feeds.conf.default: remove freifunk feed

The freifunk feed is being removed becasue
a) it is an external project and the OpenWrt team does not have access to it.
b) upon original addition of the feed, there was only a very weak tendency for
the addition.
c) there is a general lack of interest in the freifunk repo to review and/or
merge pull requests.
d) as far as can be found, all projects which use the freifunk feed have their
own make system and self-maintained feeds list.  They do not use the
feeds.conf.default from the openwrt repo.

more information can be read at the following links:

http://lists.openwrt.org/pipermail/openwrt-devel/2021-February/033807.html
https://github.com/freifunk/openwrt-packages/issues/37



Signed-off-by: Perry Melange <isprotejesvalkata@gmail.com>