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>

Documentation/kernel-parameters.txt

@@ -655,11 +655,6 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	dscc4.setup=	[NET]
 
-	dynamic_printk	Enables pr_debug()/dev_dbg() calls if
-			CONFIG_DYNAMIC_PRINTK_DEBUG has been enabled.
-			These can also be switched on/off via
-			<debugfs>/dynamic_printk/modules
-
 	earlycon=	[KNL] Output early console device and options.
 		uart[8250],io,<addr>[,options]
 		uart[8250],mmio,<addr>[,options]