Removes security deprecated code:
What does this MR do?
After https://gitlab.com/gitlab-org/release-tools/-/issues/369, we now
validate and merge MR's in batches, making MergeRequestsMerger
class
deprecated. This commit removes the service and
its spec, and implicitly the feature flag security_merge_in_batches
Related to https://gitlab.com/gitlab-org/release-tools/-/issues/427 and to https://gitlab.com/gitlab-org/release-tools/-/issues/428
Edited by Mayra Cabrera