vm_object: Release object swap charge in the swap pager destructor

With the removal of OBJT_DEFAULT, we can simply handle this in
swap_pager_dealloc(). No functional change intended.

Suggested by: alc
Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35787