Fix bug in Merge When Checks Pass needing a pipeline

What does this MR do and why?

Merge When Checks Pass should function when a pipeline does not exist, and is not needed. Currently we inherit from MergeWhenPipelineSucceeds which requires a pipeline.

To test:

• Have a project which does not use pipelines and the setting 'pipeline required to pass' is not checked
• Ensure merge_when_checks_pass and additional_merge_when_checks_ready feature flags are on
• Be on master
• Create an MR that is mergeable except for an approval
• Click Auto Merge
• Click Approve
• It will fail to merge
• Switch to this branch
• Unapprove
• Cancel auto merge
• Click Auto merge
• Approve again
• It should merge fine

Related to: #441316 (closed)