Remove TensorFlow dependency

[alecandido]

TensorFlow is only required for the classifiers, but it has been shown by @Edoardo-Pedicillo that the classifiers available in scikit-learn are already good enough, at least for the time being.

TensorFlow is a complex dependency, with multiple sub-packages, and not always promptly distributing wheels (and possibly not for all platforms), unlike NumPy, e.g..

Most likely, upgrading to the last release candidate could be enough, as it will already be in Qibo.
qiboteam/qibo#1231 (comment)
But if it is not strictly needed, I'd make our lives easier and drop it, together with the related code.

Note

I'm not suggesting to drop the classifiers and their infrastructure at all. The qubit fit and the scikit-learn ones are not causing any issue, and they could be very valuable.

[rscircus]

+1 - we also have issues with them all the time + google product half-life is low