netfilter: revert introduction of egress hook

This reverts the following commits:

  8537f786 ("netfilter: Introduce egress hook")
  5418d388 ("netfilter: Generalize ingress hook")
  b030f194 ("netfilter: Rename ingress hook include file")

>From the discussion in [0], the author's main motivation to add a hook
in fast path is for an out of tree kernel module, which is a red flag
to begin with. Other mentioned potential use cases like NAT{64,46}
is on future extensions w/o concrete code in the tree yet. Revert as
suggested [1] given the weak justification to add more hooks to critical
fast-path.

  [0] https://lore.kernel.org/netdev/cover.1583927267.git.lukas@wunner.de/
  [1] https://lore.kernel.org/netdev/20200318.011152.72770718915606186.davem@davemloft.net/



Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Miller <davem@davemloft.net>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Nacked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>