[flang] Remove unused using decls (NFC)

Identified with misc-unused-using-decls.

flang/tools/flang-driver/driver.cpp

@@ -32,8 +32,6 @@
 #include "llvm/TargetParser/Host.h"
 #include <stdlib.h>
 
-using llvm::StringRef;
-
 // main frontend method. Lives inside fc1_main.cpp
 extern int fc1_main(llvm::ArrayRef<const char *> argv, const char *argv0);
 

flang/unittests/Optimizer/InternalNamesTest.cpp

@@ -13,7 +13,6 @@
 
 using namespace fir;
 using llvm::SmallVector;
-using llvm::StringRef;
 
 struct DeconstructedName {
   DeconstructedName(llvm::StringRef name) : name{name} {}