Reader: remove unused files

[ScoutHarris]

Fixes #n/a
Ref: #15382

This removes all (as far as I can tell) unused Reader files. Specifically:

• ReaderSavedPostsViewController
• ReaderSavedPostCellActions
• ReaderMenuViewController
• ReaderFollowedSitesStreamHeader
• ReaderMenuViewModel and related classes.
  • DiscoverMenuItemCreator
  • FollowingMenuItemCreator
  • LikedMenuItemCreator
  • OtherMenuItemCreator
  • ReaderMenuItemCreator
  • SavedForLaterMenuItemCreator
  • SearchMenuItemCreator

There is one more final cleanup step to do in a future PR, and that is to address the 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.
• Verify the unit and UI tests pass on this PR.

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]

	Warnings
⚠️	PR has more than 500 lines of code changing. Consider splitting into smaller PRs if possible.

Generated by 🚫 dangerJS

[peril-wordpress-mobile]

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

[mindgraffiti]

Smoke-tested likes, commenting, saved posts, reblogging, navigating deeper into the levels of Reader from different origins, followed / unfollowed sites and checked out a few action sheet items. LGTM!

[ScoutHarris]

Thanks @mindgraffiti !