Drop all merge requests from merge trains when the project-level setting is disabled

Problem

Currently, when the project-level option is disabled, the system drops MRs from the merge trains when a pipeline finished, however, we should drop them right after the option is disabled i.e. executing AutoMergeProcessWorker immediately when the option is turned off.

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/67240

Useful: #12996 (closed)