dnsmasq: drop dnssec timestamp file patch

dnsmasq: drop dnssec timestamp file patch

Openwrt no longer uses and has not used since 5acfe55d71 Jun 2016 the
timestamp file (/etc/dnsmasq.time) method of resolving the dnssec/ntp
dnslookup chicken/egg problem, having used signals from ntp since that
change.

Drop the 'dnssec-improve-timestamp-heuristic' patch since it is neither
used nor sent upstream.  One less thing to refresh & maintain.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(backported from 7b083bbb825029ef8dd72c676a1eabae61627573)