Remove multi-backend Keras. (#1045)

From the keras repo: https://github.com/keras-team/keras/tree/2.4.0

> Multi-backend Keras has been discontinued. At this time, we recommend that Keras users who use multi-backend Keras with the TensorFlow backend switch to tf.keras in TensorFlow 2.0.

http:https://b/184083722

Dockerfile

@@ -73,7 +73,6 @@ RUN apt-get install -y libfreetype6-dev && \
  # Pinned to match GPU version. Update version together.
  pip install lightgbm==3.2.1 && \
  pip install pydot && \
- pip install keras && \
  pip install keras-tuner && \
  pip install flake8 && \
  # Pinned because it breaks theano test with the latest version (b/178107003).
@@ -248,10 +247,6 @@ RUN pip install tensorpack && \
 RUN pip install --upgrade cython && \
  pip install --upgrade cysignals && \
  pip install pyfasttext && \
- # ktext has an explicit dependency on Keras 2.2.4 which is not
- # compatible with TensorFlow 2.0 (support was added in Keras 2.3.0).
- # Add the package back once it is fixed upstream.
- # pip install ktext && \
  pip install fasttext && \
  apt-get install -y libhunspell-dev && pip install hunspell && \
  pip install annoy && \
@@ -450,7 +445,7 @@ ENV PYTHONPATH=$PYTHONPATH:/opt/facets/facets_overview/python/
 ENV MKL_THREADING_LAYER=GNU
 
 # Temporary fixes and patches
- # Temporary patch for Dask getting downgraded, which breaks Keras
+# Temporary patch for Dask getting downgraded, which breaks Keras
 RUN pip install --upgrade dask && \
  # Stop jupyter nbconvert trying to rewrite its folder hierarchy
  mkdir -p /root/.jupyter && touch /root/.jupyter/jupyter_nbconvert_config.py && touch /root/.jupyter/migrated && \