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

What does this MR do?

This MR removes the codequality_mr_diff feature flag (#284140 (closed)) - this flag is enabled by default and can be removed now.

Technically, this MR:

• Removes the .yml file that defines the feature flag
• Removes checks for this feature flag from the codebase
• Removes specs that checked for behavior when the flag was disabled
• Removes instructions on how to disable the flag from the docs

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.