swap_pager: Removing handling for objects with OBJ_SWAP clear

With the removal of OBJT_DEFAULT, we can assume that pager operations
provide an object with OBJ_SWAP set. Also, we do not need to convert
objects from type OBJT_DEFAULT. Thus, remove checks for OBJ_SWAP and
remove code which modifies the object type. In some places, replace the
check for OBJ_SWAP with a check for whether any swap blocks are
assigned.

Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35786