rmap: remove anon_vma_link() nommu stub

anon_vma_link() is unused since commit 5beb4930 ("mm: change anon_vma
linking to fix multi-process server scalability issue").

Link: https://lkml.kernel.org/r/cdce9b00c9ab15f6d02eddf40dcad537d3e9676f.1692877089.git.baruch@tkos.co.il


Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>