src: revert broken reject icmp code support

src: revert broken reject icmp code support

This patch reverts Alvaro's 34040b1 ("reject: add ICMP code parameter
for indicating the type of error") and 11b2bb2 ("reject: Use protocol
context for indicating the reject type").

These patches are flawed by two things:

1) IPv6 support is broken, only ICMP codes are considered.
2) If you don't specify any transport context, the utility exits without
   adding the rule, eg. nft add rule ip filter input reject.

The kernel is also flawed when it comes to the inet table. Let's revert
this until we can provide decent reject reason support.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>