prosody: use default network, remove libevent, config deprecated

`use_libevent` config option has been deprecated as there are 3
networking back-ends (see: https://prosody.im/doc/network_backend). It
is probably unwise to stray from the defaults (epoll).

Review instructed to flat-out remove it versus previously making it opt-in.

nixos/modules/services/networking/prosody.nix

···
 
       admins = ${toLua cfg.admins}
 
-      -- we already build with libevent, so we can just enable it for a more performant server
-      use_libevent = true
-
       modules_enabled = {
 
         ${ lib.concatStringsSep "\n  " (lib.mapAttrsToList

pkgs/servers/xmpp/prosody/default.nix

···
 , icu
 , lua
 , nixosTests
-, withLibevent ? true
 , withDBI ? true
 # use withExtraLibs to add additional dependencies of community modules
 , withExtraLibs ? [ ]
···
   luaEnv = lua.withPackages(p: with p; [
       luasocket luasec luaexpat luafilesystem luabitop luadbi-sqlite3 luaunbound
     ]
-    ++ lib.optional withLibevent p.luaevent
     ++ lib.optional withDBI p.luadbi
     ++ withExtraLuaPackages p
   );