frontend: remove all APIv1 code
According to the deprecation schedule
https://fedora-copr.github.io/posts/EOL-APIv1-APIv2
we just took the following step:
> September 2021 - remove APIv1 code from the frontend. I.e., Copr
service will not understand APIv1 calls.
The APIv1 code was removed from copr-cli and python3-copr months ago.