sched: remove long deprecated CLONE_STOPPED flag

This warning was added in commit bdff746a ("clone: prepare to recycle
CLONE_STOPPED") three years ago.  2.6.26 came and went.  As far as I know,
no-one is actually using CLONE_STOPPED.

Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>