thread_info: Remove legacy arg0-3 from restart_block

posix timers were the last users of the legacy arg0-3 members of
restart_block. Remove the cruft.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Tested-by: Richard Cochran <richard.cochran@omicron.at>
LKML-Reference: <20110201134418.326209775@linutronix.de>