Revert "Merge branch '28682-can-merge-branch-before-build-is-started' into 'master'"

This reverts commit 793be43b, reversing changes made to 8d0b4872.

For projects not using any CI, enabling merge only when pipeline succeeds caused merge requests to be in unmergeable state, which caused significant confusion.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/55144 for more details.