[libc++] Remove unneeded qualifier.

In D117811 @Quuxplusone pointed out the friend declarations don't need
to be qualified. Removing the qualification should avoid needing to add
a GCC work-around when changing _VSTD to std.

Reviewed By: Quuxplusone, philnik, #libc, ldionne

Differential Revision: https://reviews.llvm.org/D118719