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

What does this MR do?

Remove ci_mini_pipeline_gl_dropdown feature flag FF, rollout issue: #300400 (closed)

☝ The main implementation changes happen in stage.vue, where the implementation of the mini pipeline mostly lives.

Solves #299899 (closed) and #299902 (closed)

Screenshots (strongly suggested)

Some screenshots for verification:

Main Pipelines Table	Pipeline Table in Commit	Pipeline Table in MR	Pipeline Table in Commit

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 #300400 (closed)