sched: simplify can_migrate_task()

Remove trivial conditional branch in Linux scheduler's
can_migrate_task() function.

   text    data     bss     dec     hex filename
   34770    2998      24   37792    93a0 sched.o.before
   34757    2998      24   37779    9393 sched.o.after

Signed-off-by: Sven-Thorsten Dietrich <sven@thebigcorporation.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>