Demo server module needs updating

[tieneupin]

The function _register_picked_particles_use_diameter was moved from murfey.server.__init__.py to its own workflow, but this was not reflected in the murfey.server.demo_api.py module. MyPy rightly throws an error.

This should be rectified in a subsequent PR.

[tieneupin]

Demo server has been removed as of PR #723 .