MINIFICPP-2172 Remove TensorFlow extension

[lordgamez]

https://issues.apache.org/jira/browse/MINIFICPP-2172

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[szaszm]

I want to clarify why it's removed, for future reference: the extension has a difficult to install dependency that prevents regular testing, and for this reason, it was not really well maintained. It doesn't even compile in its current state, and we see no reason to keep dragging forward this mostly dead extension. Users are still free to use tensorflow through python scripting, which is a more popular way to use the library anyway.