Don't need giant for these drivers dev nodes.

Also, Giant isn't required to busy / unbusy a device, so drop that too while I'm
here. It's not done elsewhere in the tree and in the future will likely be
handled by a node lock to ensure consistency. Leave Giant in place for attach
and removing childing, as that's actually still needed, even if imperfect.

Remove stale comment about contigmalloc taking Giant and calling w/o the lock
held. Neither of these is still true.