Revert "Optimistic locking for Issue and Merge Requests"

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/models/concerns/issuable.rb

app/views/shared/issuable/_form.html.haml

db/migrate/20160707104333_add_lock_to_issuables.rb

db/schema.rb

features/project/merge_requests.feature

spec/features/issues_spec.rb

spec/features/merge_requests/edit_mr_spec.rb