Optimise runner update cached info

What does this MR do?

This tries to resolve this concern (that is part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18901):

Are these validations going to cause any serious performance problems? I think we save a runner every time contacted_at changes and we are loading these relations every time. Seems problematic? Are there other such instances these validations will trigger since we assume that runners are saved infrequently otherwise.

The change removes validation for cached data, as this is not really needed in that context.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by 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
• Conform by the merge request performance guides
• Conform by the style 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?