[lldb][RISC-V] Remove unused variable

Summary: Added in a1ffabc.

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Differential Revision: https://phabricator.intern.facebook.com/D60251580

Author: DavidSpickett

lldb/source/Plugins/Instruction/RISCV/EmulateInstructionRISCV.cpp

@@ -622,8 +622,6 @@ std::optional<DecodeResult> EmulateInstructionRISCV::Decode(uint32_t inst) {
   Log *log = GetLog(LLDBLog::Unwind);
 
   uint16_t try_rvc = uint16_t(inst & 0x0000ffff);
-  // check whether the compressed encode could be valid
-  uint16_t mask = try_rvc & 0b11;
   uint8_t inst_type = RV64;
 
   // Try to get size of RISCV instruction.