[Feature flag] Rollout of "vue_notification_dropdown" [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR removes the :vue_notification_dropdown feature flag along with a bunch of deprecated/unused code. Now that we have the Vue notifications component (which got introduced in !52068 (merged), !53045 (merged) and !52809 (merged)), we no longer need the Rails controller and the HAML templates.

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #321372 (closed)