[VPlan] Remove unused removeLastOperand (NFC).

The last use of the function has been removed a while ago. Remove the
unused function.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanValue.h

@@ -261,11 +261,6 @@ class VPUser {
     New->addUser(*this);
   }
 
-  void removeLastOperand() {
-    VPValue *Op = Operands.pop_back_val();
-    Op->removeUser(*this);
-  }
-
   typedef SmallVectorImpl<VPValue *>::iterator operand_iterator;
   typedef SmallVectorImpl<VPValue *>::const_iterator const_operand_iterator;
   typedef iterator_range<operand_iterator> operand_range;