cmd/pubsubhelper: drop USP parameter from Gerrit CL URLs
cmd/pubsubhelper: drop USP parameter from Gerrit CL URLs

Earlier this month¹, Gerrit started to include a USP query parameter in
the X-Gerrit-ChangeURL email header. Since it's not useful to propagate
it to the GerritEvent, drop it.

Two years after pubsubhelper started computing the Gerrit project name²
a dedicated header in the email was added³, so also simplify that code
while here.

¹ https://gerrit-review.googlesource.com/c/gerrit/+/340343
² https://go.dev/cl/39551
³ https://gerrit-review.googlesource.com/c/gerrit/+/247196

Fixes golang/go#54031.

Change-Id: Ic737bfdae9737851d385b5d80f32bb38760a6a9e
Reviewed-on: https://go-review.googlesource.com/c/build/+/419374
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build