Switch x11-fonts/xorg-fonts-* to USES=metaport

Also, remove bugus CONFLICTS.

I believe that the FONTDIR could be renamed to _FONTDIR and the RUN_DEPENDS
sorted, but let's do it another time to keep the diff small. Also,
prefixing non-standard variables with "_" is not officially documented yet
in the Porter's Handbook (it's only the convention suggested by portclippy,
which itself is not mentioned there yet).

Reviewed by: zeising (earlier version)
Approved by: portmgr blanket
Differential Revision: https://reviews.freebsd.org/D26989