Apply branch rule default for deleting source branch on merge requests

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When creating a merge request the default setting for if the source branch should be deleted after merging is controlled by projects.remove_source_branch_after_merge.

A branch specific setting is created as part of #504648

The branch level setting should be applicable at least when:

• creating merge requests that are targeting the configured protected branch (https://gitlab.com/gitlab-org/gitlab/-/blob/5001d20f96a2c2b684cd4afd1e91521c2de01712/app/services/merge_requests/build_service.rb#L57-63)
• when merging merge requests (https://gitlab.com/gitlab-org/gitlab/-/blob/5001d20f96a2c2b684cd4afd1e91521c2de01712/app/services/merge_requests/merge_service.rb#L126-129)