Get rid of unnecessary commit action in pipelines controller

What does this MR do?

Removes the unnecessary creation of a instance variable @commit from the pipelines_controller.

Why was this MR needed?

Less code is better code. DZ: Also see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20980?w=1&view=parallel#note_92097160

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• None