Refactor runner resource class

What does this MR do and why?

Describe in detail what your merge request does and why.

Tidying up the class a bit:

• Changed to use attributes like other resource classes
• Moved finding the runner out of remove_via_api! since it just doesn't make sense to be there.
• Allowed to repopulate resource attributes (for future purposes like checking if status has changed via API request etc...)
• The only affected test is api/4_verify/remove_runner_spec.rb, everything else should work as is.
• This MR is needed before !105570 (merged)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.