[libc++] Remove unnecessary MSVCRT exclusions

[smeenai]

Summary

Visual Studio 2013 and up have these functions, and we don't need to
support older versions.

There are some remaining _LIBCPP_MSVCRT exclusions which are present on
Visual Studio 2015 but not 2013. Those will be addressed in a follow-up.

[smeenai]

Fixing some more exclusions

include/cstdio

 using ::fflush;
 using ::setbuf;
 using ::setvbuf;
 using ::fprintf;
 using ::fscanf;
 using ::snprintf;
 using ::sprintf;
 using ::sscanf;
-#ifndef _LIBCPP_MSVCRT
 using ::vfprintf;
 using ::vfscanf;
 using ::vsscanf;
-#endif // _LIBCPP_MSVCRT
 using ::vsnprintf;
 using ::vsprintf;
 using ::fgetc;
 using ::fgets;
 using ::fputc;
 using ::fputs;
 using ::getc;
 using ::putc;