[libc++] Remove invalid use of `#if _LIBCPP_STD_VER >= 11`, as...

[libc++] Remove invalid use of `#if _LIBCPP_STD_VER >= 11`, as `_LIBCPP_STD_VER` can never be less than 11.

The relevant part of `__config` is:
```
#ifndef _LIBCPP_STD_VER
#  if  __cplusplus <= 201103L
#    define _LIBCPP_STD_VER 11
```

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D93025