Prior to r329071, pmap_bootstrap() used pmap_kmem_choose() to round the
first available virtual address to a 2MB boundary. After r329071,
create_pagetables() rounds firstaddr up to a 2MB boundary. This ensures
the kernel is mapped in super-pages, which is the point of the logic
in pmap_kmem_choose(). Therefore, it is no longer necessary for
pmap_bootstrap() to round up to the 2MB boundary again.

As pmap_bootstrap() was the only user of pmap_kmem_choose(), we can
delete pmap_kmem_choose().