[AMDGPU] Assembler: remove unused AMDGPUMCObjectWriter.

Summary: It is replaced by AMDGPUELFObjectWriter

Reviewers: tstellarAMD, vpykhtin, artem.tamazov

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl

Differential Revision: https://reviews.llvm.org/D24654

llvm-svn: 282065