Clean up branches in nest-simulator

[clinssen]

There seem to be a number of temporary branches that can be removed, such as

• fix-1326-hike-cmake
• releases and releases_
• s3_test
• terhorstd-issue-templates
• travis_deploy_fix

Perhaps more importantly, there are branches with names similar to release tags:

• nest-2.16
• nest-2.18
• nest-2.20

These are however independent from and not generally equivalent to the corresponding releases, so to avoid users from checking out the branch rather than the tag, I suggest removal.

[github-actions[bot]]

Issue automatically marked stale!

[lekshmideepu]

Already removed the branches:

• fix-1326-hike-cmake
• s3_test
• terhorstd-issue-templates
• travis_deploy_fix
• nest-2.16
• nest-2.18
• nest-2.20

But I feel there are other branches like fix-issue*, static build etc. which should also be removed. Regarding the branch, releases and releases_ probably @heplesser and @terhorstd could help?