Removed commit guidelines for migrations

At some point we decided this isn't really necessary, so let's get rid
of it.

doc/development/migration_style_guide.md

@@ -35,13 +35,6 @@ Please don't depend on GitLab-specific code since it can change in future
 versions. If needed copy-paste GitLab code into the migration to make it forward
 compatible.
 
-## Commit Guidelines
-
-Each migration **must** be added in its own commit with a descriptive commit
-message. If a commit adds a migration it _should only_ include the migration and
-any corresponding changes to `db/schema.rb`. This makes it easy to revert a
-database migration without accidentally reverting other changes.
-
 ## Downtime Tagging
 
 Every migration must specify if it requires downtime or not, and if it should