Deprecate two HTTP/2 push related histograms

HTTP/2 server push is disabled. Deprecate Net.SpdyPushedStreamFate and
Net.PushedStreamVaryResponseHeader histograms.

Bug: 1426477
Change-Id: I03fd35d76a44f9673e4a81d8a9753cf8e8406da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4559206


Reviewed-by: David Schinazi <dschinazi@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149139}