Remove unmapped/freed memory regions from the object property list.

This commit fixes the following issues:
- Any unmapped/free'd memory regions (MemoryRegion instances) are not
  removed from the object property linked list of its owner (which is
  always qdev_get_machine(uc)). This issue makes adding new memory
  mapping by calling mem_map() or mem_map_ptr() slower as more and more
  memory pages are mapped and unmapped - yes, even if those memory pages
  are unmapped, they still impact the speed of future memory page
  mappings due to this issue.
- FlatView is not reconstructed after a memory region is freed during
  unmapping, which leads to a use-after-free the next time a new memory
  region is mapped in address_space_update_topology().

qemu/header_gen.py

@@ -2182,7 +2182,6 @@ symbols = (
     'object_property_allow_set_link',
     'object_property_del',
     'object_property_del_all',
-    'object_property_del_child',
     'object_property_find',
     'object_property_get',
     'object_property_get_bool',