fixup! [RISCV][NFCI] Rationalize Immediate Definitions

Author: lenary

llvm/lib/Target/RISCV/RISCVInstrInfo.td

@@ -277,12 +277,6 @@ def simm13_lsb0 : Operand<OtherVT> {
   let OperandType = "OPERAND_PCREL";
 }
 
-class UImm20Operand : RISCVOp {
-  let EncoderMethod = "getImmOpValue";
-  let DecoderMethod = "decodeUImmOperand<20>";
-  let OperandType = "OPERAND_UIMM20";
-}
-
 class UImm20OperandMaybeSym : RISCVUImmOp<20> {
   let MCOperandPredicate = [{
     int64_t Imm;