Remove newReaderNavigation feature flag.

[ScoutHarris]

Fixes #n/a

This removes the newReaderNavigation feature flag, all associated checks, and unused code.

There is more cleanup that will be done in follow-up PRs. Namely:

• Remove unused files.
• Address TODO: - READERNAVs.

To test:

There should be no functional changes, so just smoke test the Reader and all the filters to ensure they behave as expected.

Note: there is an existing issue with unsaving posts from the Saved filter.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[aerych]

Hi @ScoutHarris!
Changes look good to me. I performed a quick smoke test of the reader and the navigation seemed to work fine.
!

[ScoutHarris]

Thanks @aerych !