Remove custom photometer handling

[dekuenstle]

At the moment, psychopy-pixx provides it's own photometer handling on top of psychopy.
The purpose was to add additional photometers easily.

• this code should be removed–––the psychopy can be extended with plugins.
• our gamma scientific photometer code contains some improvements (e.g., higher messenging frequency) which should be backported to the psychopy-gamma package.