Clean up remote branches

In the olden days, we only had two kinds of branches:
* official branches (master, maintenance_XY, BETA_JAVAYZ)
* rare "private" branches following the pattern _username_/something, e.g., https://github.com/eclipse-jdt/eclipse.jdt.core/tree/manoj/experiment_1
* at some point I had several branches sherrmann/xyz, which are not to be found on github, perhaps they got pruned during the migration to github?

Meanwhile we have lots of remote branches with unclear purpose, some of which may have been simple mistakes while creating a PR (pushing to origin instead of someone's fork).

All this makes browsing the list of branches quite painful. To make things worse, github doesn't even support the hierarchical view that EGit offers, so _username_/something is no longer very helpful, when looking in github.

Frankly, I don't know who has the power to remove any such branches, but I'd appreciate some spring cleaning here.

**Caveat:** there might be branches that are subject to current PR work, like https://github.com/eclipse-jdt/eclipse.jdt.core/tree/aioob-in-polytype

Start by reviewing the repository's remote branch list on GitHub and compare it with the official, private, and potentially obsolete branch patterns described in the issue. Check whether aioob-in-polytype or other branches are tied to current PR work before proposing removals. Done means obsolete branches are identified and safely cleaned up without affecting active work.

Written by the indexing model from the issue text.