[flang][NFC] Fix build errors for clang-10

This patch fixes one worning. Since Flang sets `-Werror`, that's
sufficient for a build to fail. As per flang/README.md, Clang-10 is one
of the officially supported compilers.

Differential Revision: https://reviews.llvm.org/D88723

flang/lib/Lower/OpenACC.cpp

@@ -99,7 +99,6 @@ static void genACC(Fortran::lower::AbstractConverter &converter,
   if (loopDirective.v == llvm::acc::ACCD_loop) {
     auto &firOpBuilder = converter.getFirOpBuilder();
     auto currentLocation = converter.getCurrentLocation();
-    llvm::ArrayRef<mlir::Type> argTy;
 
     // Add attribute extracted from clauses.
     const auto &accClauseList =