Masterbar: Remove unused RTL_Admin_Bar class

[obenland]

It looks like RTL_Admin_Bar has been defunct since #11088 moved the initialization to admin_bar_init.

Fixes #17773.

Changes proposed in this Pull Request:

• Removes RTL_Admin_Bar class.
• Removes filter callback and hook that activated the class.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• Navigate to https://wordpress.com/me/account and select a RTL language like Hebrew.
• In Jetpack::get_connected_user_data() comment out the line that returns $cached_user_data.
• On your Jetpack site under Settings > General, set your site language to a RTL language like Hebrew.
• Reload wp-admin and make sure the masterbar renders in RTL.

Proposed changelog entry for your changes:

• Removed unused class RTL_Admin_Bar.

[jetpackbot]

Scheduled Jetpack release: December 1, 2020.
Scheduled code freeze: November 23, 2020

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-17781

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

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