[MC] Remove unneeded MC*AsmBackend::fixupNeedsRelaxation overrides

Follow-up to 88c0a82
("[MC] Make MCAsmBackend::fixupNeedsRelaxation not pure virtual").

Author: MaskRay

llvm/lib/Target/PowerPC/MCTargetDesc/PPCAsmBackend.cpp

@@ -191,14 +191,6 @@ class PPCAsmBackend : public MCAsmBackend {
     }
   }
 
-  bool fixupNeedsRelaxation(const MCFixup &Fixup,
-                            uint64_t Value,
-                            const MCRelaxableFragment *DF,
-                            const MCAsmLayout &Layout) const override {
-    // FIXME.
-    llvm_unreachable("relaxInstruction() unimplemented");
-  }
-
   void relaxInstruction(MCInst &Inst,
                         const MCSubtargetInfo &STI) const override {
     // FIXME.

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

@@ -68,12 +68,6 @@ class RISCVAsmBackend : public MCAsmBackend {
                              const MCValue &Target,
                              const MCSubtargetInfo *STI) override;
 
-  bool fixupNeedsRelaxation(const MCFixup &Fixup, uint64_t Value,
-                            const MCRelaxableFragment *DF,
-                            const MCAsmLayout &Layout) const override {
-    llvm_unreachable("Handled by fixupNeedsRelaxationAdvanced");
-  }
-
   bool fixupNeedsRelaxationAdvanced(const MCAssembler &Asm,
                                     const MCFixup &Fixup, bool Resolved,
                                     uint64_t Value,