Remove any unnecessary branches that we won't need later.

[bnord]

There is a dev branch and two deprecated branches. Do we need those?

[samueldmcdermott]

for the deprecated branches -- I think @evavagiakis wanted to keep them since they initiated work on some problems of future interest. One solution could be to close them now and reopen them later if needed (I believe it's always possible to reopen branches).

regarding dev: we're (mostly) following https://nvie.com/posts/a-successful-git-branching-model/ so we are using dev for development and pushing to main less frequently than otherwise. It's intended to increase reliability. I thought we had agreed on this model but we can drop dev if you feel strongly @bnord

[samueldmcdermott]

update to add that I was incorrect above about reopening branches -- that is not possible in general, and once we close them the only way to recover will be to return to their final commits. One possible suggestion is therefore to push a final commit to each branch that is easy to search in history for (add some documentation describing what they do to their readme files, for example) and then delete