sentinel-proxy: remove /etc/config folder from the package

In the version bump sentinel-proxy there was removed uci file which should be in /etc/config. This seems like somebody forget to remove it from the Makefile. It failed with this error:

install: cannot stat 'files/uci': No such file or directory

Fixes: d5ac0320 ("sentinel-proxy: Release v1.3 (device token)")

I suppose it should work even though I haven't done compile neither run test.