Remove requirements-dev.txt.

It was generated once, likely in a non-reproducible way (pip freeze
in a contributor's local environment), and never updated.

requirements-dev.txt

-absl-py==0.8.0
-astor==0.8.0
-atomicwrites==1.3.0
-attrs==19.2.0
-boto3==1.9.243
-botocore==1.12.243
-certifi==2019.9.11
-chardet==3.0.4
-Click==7.0
-docutils==0.15.2
-gast==0.2.2
-google-pasta==0.1.7
-grpcio==1.24.1
-h5py==2.10.0
-idna==2.8
-importlib-metadata==0.23
-jmespath==0.9.4
-joblib==0.14.0
-Keras-Applications==1.0.8
-Keras-Preprocessing==1.1.0
-Markdown==3.1.1
-more-itertools==7.2.0
-numpy==1.17.2
-opt-einsum==3.1.0
-packaging==19.2
-pluggy==0.13.0
-protobuf==3.10.0
-py==1.8.0
-pyparsing==2.4.2
-pytest==5.2.1
-python-dateutil==2.8.0
-regex==2019.8.19
-requests==2.22.0
-s3transfer==0.2.1
-sacremoses==0.0.35
-sentencepiece==0.1.83
-six==1.12.0
-tensorboard==2.0.0
-tensorflow==2.0.0
-tensorflow-estimator==2.0.0
-termcolor==1.1.0
-torch==1.2.0
-tqdm==4.36.1
-urllib3==1.25.6
-wcwidth==0.1.7
-Werkzeug==0.16.0
-wrapt==1.11.2
-zipp==0.6.0