Documentation/kernel-parameters: Remove obsolete ip2=

The handling of ip2= in drivers/char/ip2/ip2base.c was moved to
drivers/char/ip2/ip2main.c in commit
47babd4c6a16915aeb15d4216d91f03910572982 ("Char: merge ip2main and
ip2base").

The ip2 driver was demoted to staging in commit
4a6514e6d096716fb7bedf238efaaca877e2a7e8 ("tty: move obsolete and broken
tty drivers to drivers/staging/tty/"), and finally deleted in commit
51c9d654c2def97827395a7fbfd0c6f865c26544 ("Staging: delete tty drivers").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>