This option does not get persisted to disk, which makes it different
from the rest of the setters. Remove it until we go all the way.

We still respect the configuration option, and it's still possible to
perform a one-time prune by calling the function.

For each remote-tracking branch we want to remove, we need to consider
it against every other refspec in case we have overlapping refspecs,
such as with

    refs/heads/*:refs/remotes/origin/*
    refs/pull/*/head:refs/remotes/origin/pr/*

as we'd otherwise remove too many refspecs.

Create a list of condidates, which are the references matching the rhs
of any active refspec and then filter that list by removing those
entries for which we find a remove reference with any active
refspec. Those which are left after this are removed.

This makes a fetch+prune more similar to a connect+prune and makes it
more likely that we see errors in the decision to prune a reference.

This is hiding a bug in the prune code, whereby we prune references we
shouldn't but don't notice it in the code afterwards because
update_tips() recreates them.

This means that we do perform changes to the references (and get rid of
the reflogs) when we shouldn't.

We load the remote again, so we need to ask the new remote to prune the
refs, or we're not exercising the code in our tests.