Remove stripped code (dotnet/coreclr#9269)

Remove stripped code (dotnet/coreclr#9269)

Delete essentially all the code that the BCL rewriter is removing.
Added back some code it shouldn't have been removing, eg., serialization code, debugger visualizers, and some changing of 'protected' to 'protected internal'

Commit migrated from https://github.com/dotnet/coreclr/commit/ee5862c6a257e60e263537d975ab6c513179d47f