[memprof] Remove an unused using directive

We've switched to LineLocation from FieldsAre, so we don't need this
"using" directive anymore.

Author: kazutakahirata

llvm/unittests/Transforms/Instrumentation/MemProfUseTest.cpp

@@ -26,7 +26,6 @@ using namespace llvm;
 using namespace llvm::memprof;
 using testing::Contains;
 using testing::ElementsAre;
-using testing::FieldsAre;
 using testing::Pair;
 using testing::SizeIs;
 using testing::UnorderedElementsAre;