pptpd: remove IP in options.pptp

ppp has obsoleted this option (as of v2.4.3-7) according to
http://wiki.openwrt.org/doc/howto/vpn.server.pptpd

Signed-off-by: Bruno Randolf <br1@einfach.org>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@38417 3c298f89-4303-0410-b956-a3cf2f4a3e73