Notification Comment Details: hide back button title in threaded comments

[ScoutHarris]

Ref: #17790 , #18049

This removes the title from the back button in the threaded comments view.

To test:

• Go to Notifications > Comments > comment details.
• Tap the header to display the threaded comments.
• Verify the nav bar back button does not have a title.

Before	After

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

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

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.

[wpmobilebot]

You can test the Jetpack changes on this Pull Request by downloading it from AppCenter here with build number: pr18131-adfaa7d. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[wpmobilebot]

You can test the WordPress changes on this Pull Request by downloading it from AppCenter here with build number: pr18131-adfaa7d. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[dvdchr]

LGTM!