Revert "iptables: fix dependency for libip6tc on IPV6"

Revert "iptables: fix dependency for libip6tc on IPV6"

This patch reverts commit 2dc1f54b1205094e7c6036cae6275d2c326bad3e as it
breaks the build for me on x86-64 if I've IPV6 support disabled. Same config
builds fine on `openwrt-18.06` branch at 55d078b2.

  $ grep IPV6 .config

  # CONFIG_KERNEL_IPV6 is not set
  # CONFIG_IPV6 is not set

Build errors out on:

  Package libiptc is missing dependencies for the following libraries:
  libip6tc.so.0

Looking at iptables-1.6.2/libiptc/Makefile.am:

  libiptc_la_LIBADD   = libip4tc.la libip6tc.la

and to iptables-1.6.2/libiptc/libiptc.pc.in:

  Requires:	libip4tc libip6tc

It seems that libiptc needs v4/v6 libs, so v6 isn't optional.

Cc: Rosy Song <rosysong@rosinson.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(backported from 1b4b942bcef8638a040788ab9ae94c66e38fd960)