[flang][acc] Remove an unused variable (#137731)

Fixes what https://github.com/llvm/llvm-project/pull/137691 introduced.
diff --git a/flang/lib/Lower/OpenACC.cpp b/flang/lib/Lower/OpenACC.cpp
index 3c42e35..06697b2 100644
--- a/flang/lib/Lower/OpenACC.cpp
+++ b/flang/lib/Lower/OpenACC.cpp
@@ -4131,9 +4131,8 @@
             },
             [&](const Fortran::parser::Name &name) {
               if (const auto *symbol = name.symbol) {
-                if (const auto *commonBlockDetails =
-                        symbol->detailsIf<
-                            Fortran::semantics::CommonBlockDetails>()) {
+                if (symbol
+                        ->detailsIf<Fortran::semantics::CommonBlockDetails>()) {
                   genCtors(operandLocation, *symbol);
                 } else {
                   TODO(operandLocation,