libstdc++: Remove duplicate using-declaration in <wchar.h>

libstdc++-v3/ChangeLog:

	* include/c_compatibility/wchar.h (fgetwc): Remove duplicate
	using-declaration.

Author: abdoei

libstdc++-v3/include/c_compatibility/wchar.h

@@ -40,7 +40,6 @@ using std::wint_t;
 using std::btowc;
 using std::wctob;
 using std::fgetwc;
-using std::fgetwc;
 using std::fgetws;
 using std::fputwc;
 using std::fputws;