Don't ELOG ipc EOF

Long description:

Receiving EOF from a client is not an error, but rather a standard way a
client may disconnect from the IPC. This should rather be logged from
a consumer of the libi3 ipc_recv_message() function as a normal client
disconnect event.

fixes #1252

To apply this patch, use:
curl http://cr.i3wm.org/patch/567/raw.patch | git am

@@ -41,7 +41,6 @@ int ipc_recv_message(int sockfd, uint32_t *message_type,

         if (n == -1)

             return -1;

         if (n == 0) {

-            ELOG("IPC: received EOF instead of reply\n");

             return -2;

         }