[AMDGPU] Assembler: remove unused AMDGPUMCObjectWriter.

[SamWot]

Summary

It is replaced by AMDGPUELFObjectWriter

[arsenm]

LGTM

llvm/trunk/lib/Target/AMDGPU/MCTargetDesc/AMDGPUAsmBackend.cpp

 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/TargetRegistry.h"
 
 using namespace llvm;
 
 namespace {
 
-class AMDGPUMCObjectWriter : public MCObjectWriter {
-public:
-  AMDGPUMCObjectWriter(raw_pwrite_stream &OS) : MCObjectWriter(OS, true) {}
-  void executePostLayoutBinding(MCAssembler &Asm,
-                                const MCAsmLayout &Layout) override {
-    //XXX: Implement if necessary.
-  }
-  void recordRelocation(MCAssembler &Asm, const MCAsmLayout &Layout,
-                        const MCFragment *Fragment, const MCFixup &Fixup,
-                        MCValue Target, bool &IsPCRel,
-                        uint64_t &FixedValue) override {
-    assert(!"Not implemented");
-  }
-
-  void writeObject(MCAssembler &Asm, const MCAsmLayout &Layout) override;
-
-};
-
 class AMDGPUAsmBackend : public MCAsmBackend {
 public:
   AMDGPUAsmBackend(const Target &T)
     : MCAsmBackend() {}
 
   unsigned getNumFixupKinds() const override { return AMDGPU::NumTargetFixupKinds; };
 
   void processFixupValue(const MCAssembler &Asm,
public:
   bool mayNeedRelaxation(const MCInst &Inst) const override { return false; }
   bool writeNopData(uint64_t Count, MCObjectWriter *OW) const override;
 
   const MCFixupKindInfo &getFixupKindInfo(MCFixupKind Kind) const override;
 };
 
 } //End anonymous namespace
 
-void AMDGPUMCObjectWriter::writeObject(MCAssembler &Asm,
-                                       const MCAsmLayout &Layout) {
-  for (MCAssembler::iterator I = Asm.begin(), E = Asm.end(); I != E; ++I) {
-    Asm.writeSectionData(&*I, Layout);
-  }
-}
-
 static unsigned getFixupKindNumBytes(unsigned Kind) {
   switch (Kind) {
   case AMDGPU::fixup_si_sopp_br:
     return 2;
   case FK_SecRel_1:
   case FK_Data_1:
     return 1;
   case FK_SecRel_2: