mod_message_log: Stop cleanly

This commit fixes the following error message when shutting down ejabberd:

    @gen_mod:stop_module_keep_config:126
    {badarg,[{mod_message_log,stop,1,
              [{file,"src/mod_message_log.erl"},{line,64}]},
             {gen_mod,stop_module_keep_config,2,
              [{file,"src/gen_mod.erl"},{line,125}]},...]}

The `mod_message_log` module doesn't use `gen_mod:get_module_proc/2` to generate process names, as only one process (per node) is started for all virtual hosts.