Drop KTR_CONTENTION.

It is incomplete, has not been adopted in the other locking primitives,
and we have other means of measuring lock contention (lock_profiling,
lockstat, KTR_LOCK). Drop it to slightly de-clutter the mutex code and
free up a precious KTR class index.

Reviewed by: jhb, mjg
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14771