Remove create_dummy from public interface

[stavros11]

However, I just noticed create_dummy is not part of the public interface...

It's true that usage in tests is limited
https://github.com/search?q=repo%3Aqiboteam%2Fqibolab+create_dummy+path%3A%2F%5Etests%5C%2F%2F&type=code
and most likely even direct usage outside Qibolab (because create_platform("dummy") is fully equivalent). But it would be breaking.

We may consider deprecating, and maintain a single access point to the dummy platform. But we need to schedule these for 0.3

Originally posted by @alecandido in #1153 (comment)