compositor: Remove shell->activate callback

It's all internal to the shell plugin now.
diff --git a/compositor/shell.c b/compositor/shell.c
index db92b5e..ee15ddf 100644
--- a/compositor/shell.c
+++ b/compositor/shell.c
@@ -861,8 +861,7 @@
 
 	focus = (struct wlsc_surface *) device->pointer_focus;
 	if (state && focus && device->grab == NULL)
-		compositor->shell->activate(compositor->shell,
-					    focus, wd, time);
+		activate(compositor->shell, focus, wd, time);
 }
 
 static void
@@ -1249,7 +1248,6 @@
 
 	memset(shell, 0, sizeof *shell);
 	shell->compositor = ec;
-	shell->shell.activate = activate;
 	shell->shell.lock = lock;
 	shell->shell.unlock = unlock;
 	shell->shell.map = map;