mm/bootmem: drop duplicated kernel-doc comments

Parts of the bootmem interfaces are duplicated in nobootmem.c along with
the kernel-doc comments. There is no point to keep two copies of the
comments, so let's drop the bootmem.c copy.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>