FreeBSD change: https://svnweb.freebsd.org/base?view=revision&revision=334695

Remove comments and assertions that are no longer valid after r330809.

r330809 replaced duplication of devdesc struct fields with an embedded copy
of the devdesc struct, to avoid fragility.  That means all the scattered
comments indicating that structs must match are no longer valid.  Likewise
asserts that attempted to mitigate some of the old fragility.