Revert "Optimistic locking for Issue and Merge Requests"

The migration to add lock_version in !5146 (merged) to every issue and merge request takes too long on GitLab.com since it has to add a default value of 0 to every row. Unfortunately, Rails 4.2 doesn't work properly if the value is left as nil; anything using optimistic locking cannot be edited.

This bug was fixed in Rails 5.0 via this commit: https://github.com/rails/rails/commit/13772bfa49325a8dc26410d2dcb555665a320f92. I suggest we revert this change for now, and when we upgrade to Rails 5.0 we can reintroduce this feature.