netfilter: conntrack: remove CONFIG_NF_CONNTRACK checks from nf_conntrack_zones.h.

nf_conntrack_zones.h was wrapped in a CONFIG_NF_CONNTRACK check in order
to fix compilation failures:

  37ee3d5b ("netfilter: nf_defrag_ipv4: fix compilation error with NF_CONNTRACK=n")

Subsequent changes mean that these failures will no longer occur and the
check is unnecessary.  Remove it.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>