[libc++][test] Remove XFAIL annotation for the remove_all() test

It passes on FreeBSD now that remove_all_impl() handles FreeBSD's
non-standard error value when opening a symlink with O_NOFOLLOW.

Author: markjdb

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.remove_all/remove_all.pass.cpp

@@ -6,8 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// XFAIL: LIBCXX-FREEBSD-FIXME
-
 // UNSUPPORTED: c++03, c++11, c++14
 // UNSUPPORTED: no-filesystem
 // UNSUPPORTED: availability-filesystem-missing