remove doc for obsolete dynamic-printk kernel-parameter

re:
  commit e9d376f0
  Author: Jason Baron <jbaron@redhat.com>
  Date:   Thu Feb 5 11:51:38 2009 -0500

    dynamic debug: combine dprintk and dynamic printk

Above commit obsoleted the kernel parameter, and even removed its
entry in kernel-parameters.txt, but got reinsterted through
0cb55ad2 ("docs: alphabetize entries in kernel-parameters.txt")

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>