Revert "ObjectiveC migrator. Migrate to instancetype return type for mehods with certain prefix selector matching their class names' suffix."

This reverts commit r187626.

It is breaking the bots.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187634 91177308-0d34-0410-b5e6-96231b3b80d8