util/RefCount, db/simple/Mount: remove obsolete libc++ workarounds

No longer a problem with NDK r16.

src/util/RefCount.hxx

@@ -42,11 +42,6 @@ class RefCount {
 	std::atomic_uint n;
 
 public:
-#ifndef _LIBCPP_VERSION
-	/* the "constexpr" is missing in libc++'s "atomic"
-	   implementation */
-	constexpr
-#endif
 	RefCount():n(1) {}
 
 	void Increment() {