remove afunix from reltool.config

When removing 'afunix' from the Makefile, mentions of afunix where left behind in reltool.config. Since the Makefile instruction triggered fetching of the afunix dependency, `rebar generate` would fail.

This change has been verified by running `make devrel DEVNODES=1`. Afunix is not used in riak at this time, so removing it is deliberate and unproblematic - it was just incompletely done.