[generic-2.4] remove nat pptp conntracking patch

[generic-2.4] remove nat pptp conntracking patch

Patch 603-netfilter_nat_pptp.patch appears to be the cause of the
longstanding brcm-2.4 dnat off-by-one port forwarding bug. Months of
testing builds with this patch removed have not been able to reproduce
the issue.

The 603 patch is rather large and we (the OpenWrt developers) have not
been able to isolate the cause of the bug within the patch. The number
of people have actually use pptp nat conntracking is far less than
number of people that use port forwarding and as a result we must drop
this patch to ensure proper behavior for the majority of users.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@17552 3c298f89-4303-0410-b956-a3cf2f4a3e73