Remove SpdySession::enable_push_.

This was always false due to
https://source.chromium.org/chromium/chromium/src/+/refs/heads/main:net/http/http_network_session.cc;l=57;drc=10ccb9ee25386acad02b6280c9213b141d42f827

Also remove a few other functions and variables that are now unused.

Also update network traffic annotations following directions at
https://chromium.googlesource.com/chromium/src/+/main/tools/traffic_annotation/scripts/auditor/README.md. Then successively make changes to annotations.xml and grouping.xml as suggested by failing tryjob outputs.

Bug: 1426477
Change-Id: Iaa602ca505622e1d19ebfaf1fe19f1260064bf23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4482906


Reviewed-by: Side YILMAZ <sideyilmaz@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148102}