Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true

The issue was that `MergeRequest#mergeable?` returns false when the CI
state is not success and project.only_allow_merge_if_build_succeeds is
true. In this case `Projects::MergeRequestsController#merge` would
return the `:failed` status when enabling `merge_when_build_succeeds`,
thus leading to a weird state and the MR never automatically merged.

The fix is to disable the CI state check in the controller safeguard
that early return the `:failed` status.

Signed-off-by: Rémy Coutable <remy@rymai.me>