Nav Block: Fix submenus being overlapped by wrapping top-level nav links.

[talldan]

Description

My previous PR removed a z-index rule from the nav block - #21033.

Some further testing shows that this resulted in top level navigation link overlapping links in submenus when a nav menu wraps (see screenshots).

This PR fixes the issue by removing another z-index rule.

How has this been tested?

1. Add a navigation block
2. Add a couple of navigation links.
3. Make the text of the first navigation link very long so that it wraps onto a second line
4. Add a submenu to the first nav link
5. Observe that the submenu overlaps the wrapped top level link

Screenshots

Before

After

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: -12 B (0%)

Total Size: 859 kB

Filename	Size	Change
build/block-library/style-rtl.css	7.43 kB	-6 B (0%)
build/block-library/style.css	7.43 kB	-6 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	998 B	0 B
build/annotations/index.js	3.43 kB	0 B
build/api-fetch/index.js	3.39 kB	0 B
build/autop/index.js	2.58 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.02 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	102 kB	0 B
build/block-editor/style-rtl.css	10.9 kB	0 B
build/block-editor/style.css	10.9 kB	0 B
build/block-library/editor-rtl.css	7.24 kB	0 B
build/block-library/editor.css	7.24 kB	0 B
build/block-library/index.js	110 kB	0 B
build/block-library/theme-rtl.css	669 B	0 B
build/block-library/theme.css	671 B	0 B
build/block-serialization-default-parser/index.js	1.65 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	57.5 kB	0 B
build/components/index.js	191 kB	0 B
build/components/style-rtl.css	15.8 kB	0 B
build/components/style.css	15.7 kB	0 B
build/compose/index.js	6.21 kB	0 B
build/core-data/index.js	10.6 kB	0 B
build/data-controls/index.js	1.04 kB	0 B
build/data/index.js	8.25 kB	0 B
build/date/index.js	5.37 kB	0 B
build/deprecated/index.js	771 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.06 kB	0 B
build/edit-post/index.js	91.2 kB	0 B
build/edit-post/style-rtl.css	8.47 kB	0 B
build/edit-post/style.css	8.46 kB	0 B
build/edit-site/index.js	6.72 kB	0 B
build/edit-site/style-rtl.css	2.88 kB	0 B
build/edit-site/style.css	2.88 kB	0 B
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build/edit-widgets/style-rtl.css	2.58 kB	0 B
build/edit-widgets/style.css	2.58 kB	0 B
build/editor/editor-styles-rtl.css	428 B	0 B
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build/editor/index.js	43.8 kB	0 B
build/editor/style-rtl.css	4 kB	0 B
build/editor/style.css	3.98 kB	0 B
build/element/index.js	4.44 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	6.95 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	1.93 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.49 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.3 kB	0 B
build/keycodes/index.js	1.69 kB	0 B
build/list-reusable-blocks/index.js	2.99 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	4.84 kB	0 B
build/notices/index.js	1.57 kB	0 B
build/nux/index.js	3.01 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.54 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	781 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.4 kB	0 B
build/server-side-render/index.js	2.55 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.01 kB	0 B
build/viewport/index.js	1.61 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

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[draganescu]

The best way to achieve perfection, making sure nothing else can be removed!