Remove support for `fl_model` module

[MaddyGuthridge]

The fl_model code is incomplete, and in many cases doesn't reflect FL Studio's behavior. In a future version, support for it will be removed.

If you depend on this code to test your script, you may prefer to use the flapi library to remotely control FL Studio. In the near future, it will support decorating other scripts in order to provide support for decorating scripts so that unit tests can be written using frameworks such as Pytest. See Flapi#4