Remove ci_pipeline_merge_request_presence_check feature flag

What does this MR do and why?

It removes the ci_pipeline_merge_request_presence_check feature flag which was introduced in %14.8 and it was enabled by default.

Related to #350734 (closed) and !78574 (merged)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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