[Etherpad] Remove deprecated user font size plugin

pica-etherpad/Dockerfile

@@ -11,7 +11,7 @@ ARG ETHERPAD_PLUGINS="\
   ep_comments_page ep_countable ep_delete_after_delay ep_delete_empty_pads ep_font_color \
   ep_font_family ep_headings2 ep_markdown ep_message_all ep_pad_activity_nofication_in_title \
   ep_pads_stats ep_prompt_for_name ep_set_title_on_pad \
-  ep_subscript_and_superscript ep_tables3 ep_user_font_size"
+  ep_subscript_and_superscript ep_tables3"
 
 RUN apt-get update && \
     apt-get install -y \