x86, apm: Remove unused variable

Commit f80c5b39 ("sched/idle, x86: Switch from TS_POLLING to
TIF_POLLING_NRFLAG") left variable "polling" unused, so remove it.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>