Rollout out `enable_inputs_for_mr_pipelines` feature flag for the MR pipeline inputs feature

What does this MR do and why?

Removes the enable_inputs_for_mr_pipelines feature flag now that the MR pipeline inputs feature has been fully rolled out and deemed stable. All feature flag checks are removed from the frontend components and backend controller/mutation, and the flag YAML definition is deleted.

References

Related to #593775 (closed)

How to set up and validate locally

1. Open a project with a .gitlab-ci.yml that defines pipeline inputs:

   spec:
     inputs:
       environment:
         description: "Target environment"
         default: "staging"
   ---
   deploy:
     script: echo "Deploying to $[[ inputs.environment ]]"
     rules:
       - if: $CI_PIPELINE_SOURCE == "merge_request_event"

2. Open a merge request and click Run pipeline — verify the "Run with modified values" option is available and inputs are shown
3. Verify the feature works without needing to enable any feature flag

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.