[CodeGen] Remove unused declaration RegisterDefIsDead

The corresponding function definition was removed by:

  commit 6325446
  Author: Evan Cheng <[email protected]>
  Date:   Wed Mar 5 00:59:57 2008 +0000

Author: kazutakahirata

llvm/include/llvm/CodeGen/LiveVariables.h

@@ -182,10 +182,6 @@ class LiveVariables : public MachineFunctionPass {
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
-  /// RegisterDefIsDead - Return true if the specified instruction defines the
-  /// specified register, but that definition is dead.
-  bool RegisterDefIsDead(MachineInstr &MI, Register Reg) const;
-
   //===--------------------------------------------------------------------===//
   //  API to update live variable information