[GC] Remove a bunch of unused complexity from Registry and RegistryParser [NFCI]

The only two Registries we have in the system are the GCStrategy and GCMetadataPrinter ones.  Registry has a bunch of problems - for instance, order of initialization is undefined - and the code was overly general for what was actually used.  I hope to completely kill Registry in the near future, but for now, just delete all the unused listener and parsing support.

llvm-svn: 257727