Remove witness directives from crossmp locking VOPs

These are of limited use since the crossmp vnode locking ops have not
actually used a lock since commit
a2d35545429117e68fbcbc68e14ad55e84265d69. We in fact require that
these operations are always issued with LK_SHARED. Additionally,
these directives can produce a false positive in certain VV_CROSSLOCK
cases which require upgrading of the covered vnode lock from shared
to exclusive.

While here, replace the runtime check of LK_SHARED with a KASSERT and
expand the check to include LK_NOWAIT, which all callers pass.

Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D35054