Get rid of references to level up/down.

In v4 syntax, that's focus parent/child.

testcases/t/157-regress-fullscreen-level-up.t

@@ -30,7 +30,7 @@ is($nodes->[0]->{fullscreen_mode}, 1, 'client fullscreen now');
 #####################################################################
 # send level up, try to un-fullscreen
 #####################################################################
-cmd 'level up';
+cmd 'focus parent';
 cmd 'fullscreen';
 
 $nodes = get_ws_content $tmp;