nfp: remove cpp mutex cache

CPP mutex cache was introduced to work around the fact that the
same host could successfully acquire a lock multiple times.  It
used to collapse multiple users to the same struct nfp_cpp_mutex
and track use count.  Unfortunately it's racy.  Since we now force
all nfp_mutex_lock() callers within the host to actually succeed
at acquiring the lock we no longer need the cache, let's remove it.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>