Remove tensorforce package. (#965)

Last usage in our platform was in October 2020.
In the past year, we have seen 14 notebooks using it only.

User wanting to use this package should use `!pip install tensorforce`
into their own kernel.

TensorForce 0.6.2 has dependencies on tensorflow==2.3.1 and
numpy==1.18.5 which is incompatible with our environment. We don't want
to constrain new release of TensorFlow for a package that is barely
being used. Hence, the reason for dropping it from our base image.

http:https://b/181584341

Dockerfile

@@ -372,8 +372,6 @@ RUN pip install flashtext && \
  pip install shap && \
  pip install ray && \
  pip install gym && \
- # b/167268016 tensorforce 0.6.6 has an explicit dependency on tensorflow 2.3.1 which is causing a downgrade.
- pip install tensorforce==0.5.5 && \
  pip install pyarabic && \
  pip install pandasql && \
  pip install tensorflow_hub && \