Remove DnsHttpssvc experiment feature related codes

The DnsHttpssvc experiment has finished. So this CL removes the
DnsHttpssvc experiment feature related codes.

HttpssvcMetrics is used to measure the metrics for the experiment.
All of the metrics for Integrity record and most of metrics for HTTPS
record will not be used. But the metrics for HTTPS record
(Net.DNS.HTTPSSVC.RecordHttps.(Secure|Insecure).ExpectNoerror.*) are
still valuable. This CL keeps HttpssvcMetrics class and the name for
HTTPS record metrics as is. And I will create another CL to rename them.

Bug: 1366422
Change-Id: Ib23d89f6af83812a77e11d2568c6c9cb936037b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3911600
Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1057309}