project.config: remove trybot-result-changers

The group description says:

"Accounts that can delete TryBot result votes to restart the TryBots."

But in fact, anyone with remove reviewer permissions can delete the vote, per http://b/245706858. And due to a config error, they're only allowed to vote -1, which I think we can safely assume nobody wants.

Change-Id: I94062a2fe89611b401017a27237f509627c9c8e5
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/456675
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>