[RISCV] remove useless code

When legality check for vectoring reduction， hasVInstructions() check be unneeded. RISCV can only loop vectorization with hasVInstructions()

Reviewed By: kito-cheng, craig.topper

Differential Revision: https://reviews.llvm.org/D125460

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -171,9 +171,6 @@ public:
 
   bool isLegalToVectorizeReduction(const RecurrenceDescriptor &RdxDesc,
                                    ElementCount VF) const {
-    if (!ST->hasVInstructions())
-      return false;
-
     if (!VF.isScalable())
       return true;