[AMDGPU] Remove an unused variable (NFC)

/llvm-project/llvm/lib/Target/AMDGPU/SIISelLowering.cpp:3416:9:
 error: unused variable 'ValVT' [-Werror,-Wunused-variable]
    EVT ValVT = VA.getValVT();
        ^
1 error generated.

Author: DamonFool

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -3413,7 +3413,6 @@ SDValue SITargetLowering::LowerFormalArguments(
       RC = &AMDGPU::SGPR_32RegClass;
     else
       llvm_unreachable("Unexpected register class in LowerFormalArguments!");
-    EVT ValVT = VA.getValVT();
 
     Reg = MF.addLiveIn(Reg, RC);
     SDValue Val = DAG.getCopyFromReg(Chain, DL, Reg, VT);