Remove disable_composer_callback feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes the disable_composer_callback feature flag which has been enabled on GitLab.com since 2021-06-17

This feature flag wrapped a callback in the package model that was throwing errors and after investigation looked to be dead code. The feature flag was a safety mechanism to disable it before removing it just in case there was something missed in the investigation.

Now that it has been disabled, the errors have stopped, and there have been no other issues with the related package and Composer code, so the callback is ready to be removed entirely.

References

See this thread and this MR description if you'd like more background and details on why the callback is not needed.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #333587 (closed)