Remove default value for lock_version

What does this MR do?

From the very beginning we added lock_version column with default value, then we reverted that MR. A bit later we added that migration again but column's default value has been removed from migration. So if you installed GitLab form master it's possible that you have default value, which caused the bug.

We don't need to change a CHANGELOG here I think.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21527 and https://dev.gitlab.org/gitlab/organization/issues/971