Remove hide-referrer plugin

This plugin was annoying but nonetheless very useful for privacy. I discover that it is totally useless since [this commit](https://github.com/ether/etherpad-lite/pull/3044/files), so... Remove it

pica-etherpad/Dockerfile

@@ -44,7 +44,6 @@ RUN curl -SL https://github.com/ether/etherpad-lite/archive/${ETHERPAD_VERSION}.
         ep_font_color \
         ep_font_family \
         ep_headings2 \
-        ep_hide_referrer \
 	ep_markdown \
         ep_pad_activity_nofication_in_title \
         ep_pads_stats \