memory_mapping: Drop qemu_get_memory_mapping() stub

dump.c:dump_init() never checked for the return code anyway.
If paging is not enabled, it will fall back to an identity map.
If paging is enabled and getting memory mapping list is not
implemented, qemu_get_guest_memory_mapping() will return an error.

Since the targets not implementing memory mapping also don't implement
dump support, we will not reach this code today and can worry about
changing cpu_paging_enabled() default when the need arises.

This allows us to drop CONFIG_HAVE_GET_MEMORY_SUPPORT.

Signed-off-by: Andreas Färber <afaerber@suse.de>
