locking/rwsem: Remove empty rwsem.h

This is a leftover from 7f26482a ("locking/percpu-rwsem: Remove the embedded rwsem")

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20210126101721.976027-1-nborisov@suse.com