[lldb][NFC] Remove WriteRegister copy-pasta from ObjectFileMachO

The function had the same name as one of the member function, so
it was just copied to all classes so that the lookup works. We
could also give the function a more better and unique name
(because it's actually printing the register value and writing
to the stream, not writing to the register).

Also removes the unused return value.

llvm-svn: 370854