tasks/exit: Remove unused task_is_dead() method

tasks/exit: Remove unused task_is_dead() method

task_is_dead() has no users since commit 43e13cc107cf
("cred: remove task_is_dead() from __task_cred() validation"), and
nobody except exit.c should rely on ->exit_state (we still have
the users which should be changed).

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20131113143614.GA10547@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>