config/prod.exs: Remove server: true

This breaks mix tasks in regular Elixir

config/prod.exs

@@ -14,7 +14,6 @@
 # manifest is generated by the mix phoenix.digest task
 # which you typically run after static files are built.
 config :pleroma, Pleroma.Web.Endpoint,
-  server: true,
   http: [port: 4000],
   protocol: "http"