games/netradiant: fix packaging on powerpc64

After the last update, this port doesn't need replacing ARCH on powerpc64 and doing it actually breaks packaging:
pkg-static: Unable to access file /wrkdirs/usr/ports/games/netradiant/work/stage/usr/local/share/netradiant/heretic2/h2data.powerpc:No such file or directory

PR: 239054
Approved by: danfe (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D20889