[Modules] Delete a bunch of complex code for ensuring visible decls in

updated decl contexts get emitted.

Since this code was added, we have newer vastly simpler code for
handling this. The code I'm removing was very expensive and also
generated unstable order of declarations which made module outputs
non-deterministic.

All of the tests continue to pass for me and I'm able to check the
difference between the .pcm files after merging modules together.

llvm-svn: 233251