Remove mention of semaphores from kernel-locking

Since the consensus seems to be to eliminate semaphores where possible,
we shouldn't be educating people about how to use them as locks.  Use
mutexes instead.  Semaphores should be described in a separate document
if we end up keeping them.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>