xtensa: use generic sched_clock()

Current xtensa implementation of sched_clock() is the same as the
generic one.  Just remove it, the weak symbol in kernel/sched_clock.c
will be used instead.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Chris Zankel <chris@zankel.net>