[flang] Fix compilation error due to variable no being used (llvm#75210)

My builds were failing because the variable 'dim' was not used. This
produced a warning, and my builds have warnings set as errors.

Author: psteinfeld

flang/lib/Optimizer/HLFIR/IR/HLFIROps.cpp

@@ -663,7 +663,6 @@ template <typename NumericalReductionOp>
 static mlir::LogicalResult
 verifyArrayAndMaskForReductionOp(NumericalReductionOp reductionOp) {
   mlir::Value array = reductionOp->getArray();
-  mlir::Value dim = reductionOp->getDim();
   mlir::Value mask = reductionOp->getMask();
 
   fir::SequenceType arrayTy =