locking: Remove rt_rwlock_is_contended().

rt_rwlock_is_contended() has no users. It makes no sense to use it as
rwlock_is_contended() because it is a sleeping lock on RT and
preemption is possible. It reports always != 0 if used by a writer and
even if there is a waiter then the lock might not be handed over if
the current owner has the highest priority.

Remove rt_rwlock_is_contended().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211129174654.668506-3-bigeasy@linutronix.de