group: fix in lv_group_remove_obj to handle deleting hidden objects correctly

Fixes: https://forum.lvgl.io/t/deleting-hidden-subobjs-in-groups-may-lead-to-crash/3590

src/lv_core/lv_group.c

@@ -158,8 +158,7 @@ void lv_group_remove_obj(lv_obj_t * obj)
 {
     lv_group_t * g = obj->group_p;
     if(g == NULL) return;
-    if(g->obj_focus == NULL) return; /*Just to be sure (Not possible if there is at least one object in the group)*/
-
+  
     /*Focus on the next object*/
     if(*g->obj_focus == obj) {
         /*If this is the only object in the group then focus to nothing.*/