[AMDGPU] Remove unused declaration selectSMRD (NFC)

The function body proper was removed on Feb 20, 2019 in commit
79b5c38.

Author: kazutakahirata

llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.h

@@ -132,7 +132,6 @@ class AMDGPUInstructionSelector final : public InstructionSelector {
   bool hasVgprParts(ArrayRef<GEPInfo> AddrInfo) const;
   void getAddrModeInfo(const MachineInstr &Load, const MachineRegisterInfo &MRI,
                        SmallVectorImpl<GEPInfo> &AddrInfo) const;
-  bool selectSMRD(MachineInstr &I, ArrayRef<GEPInfo> AddrInfo) const;
 
   void initM0(MachineInstr &I) const;
   bool selectG_LOAD_STORE_ATOMICRMW(MachineInstr &I) const;