Get rid of nhidden in group_ctx; it actually never reported correctly

since nhidden wasn't incremented nor decremeted in all the right places,
thus confusing matters.  We don't need to carry a count around, so just
use a local variable in the one place we need one to supply
XRestackWindows().

calmwm.h

@@ -205,7 +205,6 @@ struct group_ctx {
 	struct client_ctx_q	 clients;
 	int			 shortcut;
 	int			 hidden;
-	int			 nhidden;
 	int			 highstack;
 };
 TAILQ_HEAD(group_ctx_q, group_ctx);