[sanitizer_common] Remove support for tirpc/rpc/xdr.h

Remove the partial support for rpc/xdr.h from libtirpc.  Since it is
an entirely external library, we ought to build it sanitized separately
and not attempt to intercept like the libc implementation.  Besides,
the existing code for tirpc support was neither complete nor working.

Noted by @krytarowski.

Differential Revision: https://reviews.llvm.org/D47817

llvm-svn: 350881