vm_mmap: Remove obsolete code and comments from vm_mmap()

In preparation for removing OBJT_DEFAULT, eliminate some stale/unhelpful
comments from vm_mmap(), and remove an unused case. In particular, the
remaining callers of vm_mmap() in the tree do not specify OBJT_DEFAULT.

It's much more common to use vm_map_find() to map an object into user
memory, so rather than adjusting vm_mmap() to handle OBJT_SWAP objects,
let's further discourage its use and simply remove OBJT_DEFAULT
handling.

Reviewed by: dougm, alc, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35778