ConnectionPool Worker: use monitor flush instead of checking ref

`:flush` removes the DOWN message if one had arrived, so this check
should no longer be necessary.

lib/pleroma/gun/connection_pool/worker.ex

@@ -93,12 +93,8 @@ def handle_call(:remove_client, {client_pid, _}, %{key: key} = state) do
       end)
 
     {ref, state} = pop_in(state.client_monitors[client_pid])
-    # DOWN message can receive right after `remove_client` call and cause worker to terminate
-    state =
-      if is_nil(ref) do
-        state
-      else
-        Process.demonitor(ref)
+
+    Process.demonitor(ref, [:flush])
 
     timer =
       if used_by == [] do
@@ -108,10 +104,7 @@ def handle_call(:remove_client, {client_pid, _}, %{key: key} = state) do
         nil
       end
 
-        %{state | timer: timer}
-      end
-
-    {:reply, :ok, state, :hibernate}
+    {:reply, :ok, %{state | timer: timer}, :hibernate}
   end
 
   @impl true