x86/Linux: reenable all-stop on top of non-stop

x86/Linux: reenable all-stop on top of non-stop

Since the record-btrace target now supports non-stop mode, we no
longer need to force-disable as-ns on x86.

gdb/ChangeLog:
2015-09-30  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_nat_always_non_stop_p): Always return 1.
	* x86-linux-nat.c (x86_linux_always_non_stop_p): Delete.
	(x86_linux_create_target): Don't install
	x86_linux_always_non_stop_p.