Documentation: Remove bootmem_debug from kernel-parameters.txt

Remove bootmem_debug kernel paramenter because it has been
replaced by memblock=debug.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/157443061745.20995.9432492850513217966.stgit@devnote2


Signed-off-by: Jonathan Corbet <corbet@lwn.net>