sched, mips, ia64: Remove __ARCH_WANT_UNLOCKED_CTXSW

Kirill found that there's a subtle race in the
__ARCH_WANT_UNLOCKED_CTXSW code, and instead of fixing it, remove the
entire exception because neither arch that uses it seems to actually
still require it.

Boot tested on mips64el (qemu) only.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kirill Tkhai <tkhai@yandex.ru>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Qais Yousef <qais.yousef@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: oleg@redhat.com
Cc: linux@roeck-us.net
Cc: linux-ia64@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Link: http://lkml.kernel.org/r/20140923150641.GH3312@worktop.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>