Revert "Merge branch '55127-add-delay-after-mr-creation-for-async-tasks-to-complete' into 'master'"

In #27978 (closed) I added a check for pipeline existence when accepting an MR via the API, as a race condition exists where rapidly creating then accepting the MR can result in the MR being merged regardless of a merge_when_pipeline_succeeds: true (MWPS) setting. After my MR was merged, @grzesiek offered additional context of a previous Issue (which we haven't been able to locate yet) which discusses what customer's expectations might be in the case where they have MWPS set to true, but a pipeline doesn't exist. There is a potential that some customer's are relying on the behavior that a true value for MWPS does not require the existence of a pipeline. @grzesiek has asked to revert #27978 (closed) until we're able to land on an app-wide solution, discussion of which can be found in #61931 (closed).