Reader Comments: Remove feature flag for Follow Conversation

[dvdchr]

Fixes n/a

As titled, this removes the feature flag for the Follow Conversation feature. This means the feature will be permanently moved from the header view to the navigation bar. In addition, I've also taken the liberty of removing unused code in Reader Comments and the header view.

To test:

Similar testing steps from #17363 should be applicable for this:

• Go to Reader, and select any unsubscribed post.
• 🔍 Verify that the "Follow" button should be visible on the navigation bar.
• Tap the Follow button.
• 🔍 Verify that the snackbar with the "Enable" action button is shown.
• Tap on the "Enable" button.
• 🔍 Verify that a feedback notice with an "Undo" action button is shown.
• Tap on the "Undo" action button.
• 🔍 Verify that a feedback notice is shown.
• 🔍 Verify that the Follow button has turned into a bell icon button in the navigation bar.
• Go to Reader, and select any subscribed post.
• Tap on the bell icon button.
• 🔍 Verify that the bottom sheet is shown.
• Tap on the switch.
• Dismiss the bottom sheet.
• 🔍 Verify that a feedback notification is shown, and correctly represents the desired state.
• Tap the bell icon button again.
• Tap on the Unfollow Conversation button.
• 🔍 Verify that a feedback notification is shown.
• 🔍 Verify that the bell icon button has now turned back into "Follow".

Regression Notes

1. Potential unintended areas of impact
   The follow conversation flow may be impacted.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested to ensure that the feature works as expected.

3. What automated tests I added (or what prevented me from doing so)
   n/a.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

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

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

[ScoutHarris]

LGTM!

[ScoutHarris]

Hey @dvdchr . FYI - I enabled auto-merge so I can use this in my feature. I also merged develop to kick off tests again. The unit tests were failing, but locally they ran fine 🤷 . 🤞 !