uma: remove UMA_ZFLAG_CACHEONLY flag

UMA_ZFLAG_CACHEONLY was essentially the same thing as UMA_ZONE_VM, but
with a more confusing name. Remove the flag, make UMA_ZONE_VM an
inherit flag, and replace all references.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23516