perf(coordinator): remove avatar generator

[Reckless-Satoshi]

What does this PR do?

Robot avatar generation has moved to the frontend since v0.6.0. There is no reason anymore for the coordinator to generate and serve avatars. In addition, current coordinator-side avatars are different to those on the clients, therefore these serve no purpose in the the coordinator panel either (only place left were these are displayed), as these backend avatar can't be used to identify a robot anymore.

This PR removes definitely robot avatar generation in the coordinator:

• Improves /api/robot auth performance, as there is no need to spend ~250-500ms assembling the .webp images.
• Improves loading speed of coordinator panel pages /coordinator/api/robot/, /coordinator/authtoken/tokenproxy/ and /coordinator/auth/user/
• Reduced compuration and storage requirements to run a RoboSats coordinator.
• Easier coordinator backups, no need to backup the /static folder anymore.

This is a breaking change. Any user still running clients v0.5.4 or earlier will not be able to keep using RoboSats anymore.

Checklist before merging

• [x] Install pre-commit and initialize it: pip install pre-commit, then pre-commit install. Pre-commit installs git hooks that automatically check the codebase. If pre-commit fails when you commit your changes, please fix the problems it points out.