Remove varobj_clear_saved_item

One call to varobj_clear_saved_item is from the varobj destructor.
This is no longer needed, so this patch removes the call; then inlines
the function into the sole remaining caller.

gdb/ChangeLog
2020-12-11  Tom Tromey  <tom@tromey.com>

	* varobj.c (varobj_clear_saved_item): Remove.
	(update_dynamic_varobj_children): Update.
	(varobj::~varobj): Don't call varobj_clear_saved_item.