[RISCV] Remove old peephole declaration in RISCVISelDAGToDAG.h. NFC

It was removed in 72e6c1c

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.h

@@ -190,7 +190,6 @@ class RISCVDAGToDAGISel : public SelectionDAGISel {
   bool doPeepholeMaskedRVV(MachineSDNode *Node);
   bool doPeepholeMergeVVMFold();
   bool doPeepholeNoRegPassThru();
-  bool performVMergeToVMv(SDNode *N);
   bool performCombineVMergeAndVOps(SDNode *N);
 };