[x86] Remove unused declaration processWaitCnt (NFC)

The declaration was introduced without a corresponding definition on
Jan 2, 2022 in commit 85e6e748d426f8992016914b07bc67c4da22e278.

GitOrigin-RevId: 54199d805a5431df292d412dbbae9286b3690ad3
diff --git a/lib/Target/X86/MCA/X86CustomBehaviour.h b/lib/Target/X86/MCA/X86CustomBehaviour.h
index 61e1b6b..ed2d540 100644
--- a/lib/Target/X86/MCA/X86CustomBehaviour.h
+++ b/lib/Target/X86/MCA/X86CustomBehaviour.h
@@ -25,8 +25,6 @@
 namespace mca {
 
 class X86InstrPostProcess : public InstrPostProcess {
-  void processWaitCnt(std::unique_ptr<Instruction> &Inst, const MCInst &MCI);
-
   /// Called within X86InstrPostProcess to specify certain instructions
   /// as load and store barriers.
   void setMemBarriers(std::unique_ptr<Instruction> &Inst, const MCInst &MCI);