Revert "Pin nltk to 3.5 (#988)" (#990)

A new version of nltk with the hotfix was released an hour ago:
- https://pypi.org/project/nltk/#history
- nltk/nltk#2693 (comment)

This reverts commit bda8a17.

Dockerfile

@@ -77,8 +77,6 @@ RUN pip install /tmp/tfa_cpu/tensorflow*.whl && \
 RUN apt-get install -y libfreetype6-dev && \
  apt-get install -y libglib2.0-0 libxext6 libsm6 libxrender1 libfontconfig1 --fix-missing && \
  pip install gensim && \
- # b/184748065 remove pin to nltk once fix has been provided for the pytest production dependency.
- pip install nltk==3.5 && \
  pip install textblob && \
  pip install wordcloud && \
  pip install xgboost && \