netfilter: conntrack: remove two unused functions from nf_conntrack_timestamp.h.

Two inline functions defined in nf_conntrack_timestamp.h,
`nf_ct_tstamp_enabled` and `nf_ct_set_tstamp`, are not called anywhere.
Remove them.

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