vm_object: Remove redundant OBJ_SWAP checks

With the removal of OBJT_DEFAULT, OBJ_ANON implies OBJ_SWAP.

Note, this means that vm_object_split() is more expensive than it used
to be, as it holds busy locks until the end of the range is reached,
even if the object has no swap blocks allocated.

Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35789