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

What does this MR do?

Remove sast_configuration_ui feature flag

A few other things were also removed as they were no longer being used:

• The create_merge_request_button.vue component, since the SAST Configuration UI would always now be available instead
• The SastConfigurationController#create action, since only the above button used that endpoint. (The configureSast GraphQL mutation is used instead of this endpoint elsewhere.)

Addresses #235135 (closed).

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] 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