memblock tests: remove completed TODO item

Remove completed item from TODO list.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Rebecca Mckeever <remckee0@gmail.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/f2263abe45613b28f1583fbf04a4bffcf735bcf6.1667802195.git.remckee0@gmail.com