Revert "Prune while fetching"

This reverts commit 17675937.  At
https://github.com/endlessm/gh_fetch/pull/3#issuecomment-507526552
@dbnicholson said:

> This defeats the purpose of the backup tool. The backup should keep
> the refs in case the remote has accidentally deleted them. For instance,
> assume that the eos3.4 branch was deleted on the remote. After this
> change, the backup will follow and we'll no longer have any reference to
> the referenced commit.
>
> In 5 years of running this code, this is the 2nd time we've ever run
> into this kind of branch conflict. I don't think we should weaken the
> backup tool to cater to it.

https://phabricator.endlessm.com/T27103