MINOR: session: clean up the connection free code

Use conn_free() instead of pool_free2(conn...). This makes the code more
auditable.
diff --git a/src/session.c b/src/session.c
index aae0c69..0a03677 100644
--- a/src/session.c
+++ b/src/session.c
@@ -314,6 +314,7 @@
 
 	/* kill the connection now */
 	conn_force_close(conn);
+	conn_free(conn);
 
 	s->fe->feconn--;
 	session_store_counters(s);
@@ -335,8 +336,6 @@
 
 	task_delete(s->task);
 	task_free(s->task);
-
-	pool_free2(pool2_connection, conn);
 	pool_free2(pool2_session, s);
 }