gitserver: remove ArchiveURL
Created by: keegancsmith
The only user of this was an internal endpoint which used to be used by zoekt. Zoekt removed its use of Sourcegraph archive a few months ago so it is no longer needed. Additionally, I confirmed on sourcegraph.com that the route had not been used in the last 30 days via the prometheus query
max_over_time(src_http_requests_last_timestamp_unixtime{route="internal.git.tar"}[30d])
Test Plan: CI