remove dependency on ip6tables

which is now (as of aosp master external/iptables @ HEAD, ie.
  https://android-review.googlesource.com/c/platform/external/iptables/+/1284244


) simply a symbolic link to iptables binary

This should fix:
  > FAILED:
  > build/make/core/main.mk:1132: warning:  build/make/target/product/generic.mk includes non-existent modules in PRODUCT_PACKAGES
  > Offending entries:
  > ip6tables
  > build/make/core/main.mk:1132: error: Build failed.

Test: No, but building, and treehugging
Bug: 154186169
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I94709a7450d0c41a64951c592b54de4b2956eae9