[gardening] Fix riscv32 build, remove unused variable.

Fixes #57021
TEST=local riscv32 build

Change-Id: I142a50a5998ec35fc40e82fd0bd2533c5f2097c4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393361
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

runtime/vm/compiler/backend/il_riscv.cc

@@ -6875,7 +6875,6 @@ void IntConverterInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
     ASSERT(to() == kUnboxedUint32 || to() == kUnboxedInt32);
     PairLocation* in_pair = locs()->in(0).AsPairLocation();
     Register in_lo = in_pair->At(0).reg();
-    Register in_hi = in_pair->At(1).reg();
     Register out = locs()->out(0).reg();
     // Copy low word.
     __ mv(out, in_lo);