experimental: remove some extraneous _LIBCPP_FUNC_VIS

These member functions were decorated with `_LIBCPP_FUNC_VIS` when the
class is also decorated with external visibility.  This breaks down when
building for PE/COFF, where the member function cannot be decorated if
it is within a decorated class.  The class attribute will propagate to
the member.  Remove the extraneous decoration.

llvm-svn: 293454