sched/x86_64: Don't save flags on context switch

Now that the kernel always runs with clean flags (in particular,
NT is clear), there is no need to save and restore flags on
every context switch.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Sebastian Lackner <sebastian@fds-team.de>
Cc: Anish Bhatt <anish@chelsio.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/bf6fb790787eb95b922157838f52712c25dda157.1412187233.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>