sched.h: Simplify HAVE_GROUP_MEMBERS logic

After the retirement of CONFIG_SCHED_EXIT_KILL_CHILDREN, [1]
the behavior is now effectively controlled by
CONFIG_SCHED_CHILD_STATUS, which is IMO confusing.
(It was even surprising me.)

This commit solves it by making HAVE_GROUP_MEMBERS
a synonym of !CONFIG_DISABLE_PTHREAD. I guess this change aligns
the intention of the CONFIG_SCHED_EXIT_KILL_CHILDREN retirement.
That is, always tries to be similar to the standard behavior.

Another option would be to revive CONFIG_SCHED_EXIT_KILL_CHILDREN
and make the HAVE_GROUP_MEMBERS logic aware of it.
It might be a good idea if we want a finer-grained control of
code to build. (One can argue his application uses pthread
but does never _exit. In that case, it might be a waste to maintain
group membership. I suspect it might not be too rare in embedded
world.)

[1] commit 6982eb35fe

include/nuttx/sched.h

@@ -51,24 +51,17 @@
 
 /* Configuration ************************************************************/
 
-/* Task groups currently only supported for retention of child status */
-
-#undef HAVE_GROUP_MEMBERS
-
-/* We need a group an group members if we are supporting the parent/child
- * relationship.
+/* We need to track group members at least for:
+ *
+ * - To signal all tasks in a group. (eg. SIGCHLD)
+ * - _exit() to collect siblings threads.
  */
 
-#if defined(CONFIG_SCHED_HAVE_PARENT) && defined(CONFIG_SCHED_CHILD_STATUS)
+#undef HAVE_GROUP_MEMBERS
+#if !defined(CONFIG_DISABLE_PTHREAD)
 #  define HAVE_GROUP_MEMBERS  1
 #endif
 
-/* We don't need group members if support for pthreads is disabled */
-
-#ifdef CONFIG_DISABLE_PTHREAD
-#  undef HAVE_GROUP_MEMBERS
-#endif
-
 /* Sporadic scheduling */
 
 #ifndef CONFIG_SCHED_SPORADIC_MAXREPL