Cleanup branches of subtreesplit managed repos

[ek9]

I've checked and some extension repositories still have branches left over. These either have to be deleted or ported over to this repository. If someone merges them to the owning repositories at some point then it might make things problematic.

Branches that needs to be removed or possibly ported over:

• (remove?) https://github.com/richardmiller/ExemplifyExtension/tree/update-to-phpspec3
• (port over?) https://github.com/richardmiller/ExemplifyExtension/tree/update-to-question-helper
• (remove?) https://github.com/richardmiller/PhpSpecRunExtension/tree/update-to-phpspec-three
• (port over?) https://github.com/richardmiller/PhpSpecExtension/tree/fix/case-mismatch

Let me know if those branches need porting over, I could do a PR. If they have already been merged an part of this repository, then feel free to remove them.