LoongArch: Remove unused relaxInstruction

Author: MaskRay

llvm/lib/Target/LoongArch/MCTargetDesc/LoongArchAsmBackend.h

@@ -59,9 +59,6 @@ class LoongArchAsmBackend : public MCAsmBackend {
 
   MCFixupKindInfo getFixupKindInfo(MCFixupKind Kind) const override;
 
-  void relaxInstruction(MCInst &Inst,
-                        const MCSubtargetInfo &STI) const override {}
-
   bool relaxDwarfLineAddr(MCDwarfLineAddrFragment &DF,
                           bool &WasRelaxed) const override;
   bool relaxDwarfCFA(MCDwarfCallFrameFragment &DF,