[Flang] Remove unnecessary static_assert

Certain compilers do not seem to like the static assert with a string, causing
a implicit conversion. It can be removed as it should not be reachable and the
mlir::failure should handle it correctly in case it is.

flang/lib/Optimizer/HLFIR/Transforms/OptimizedBufferization.cpp

@@ -750,7 +750,6 @@ class ReductionElementalConversion : public mlir::OpRewritePattern<Op> {
                                                      reduction);
       };
     } else {
-      static_assert("Expected Op to be handled");
       return mlir::failure();
     }