Remove unused comments related to Python 2 compatibility.

PiperOrigin-RevId: 437767104

research/object_detection/models/ssd_resnet_v1_fpn_feature_extractor_testbase.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/models/ssd_resnet_v1_fpn_keras_feature_extractor.py

-# Lint as: python2, python3
 # Copyright 2019 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/models/ssd_resnet_v1_ppn_feature_extractor.py

-# Lint as: python2, python3
 # Copyright 2018 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/convolutional_box_predictor.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/convolutional_box_predictor_tf1_test.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/convolutional_keras_box_predictor.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/heads/keras_mask_head.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/heads/keypoint_head.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/predictors/heads/mask_head.py

-# Lint as: python2, python3
 # Copyright 2017 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/tpu_exporters/utils_test.py

-# Lint as: python2, python3
 # Copyright 2019 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");

research/object_detection/utils/colab_utils.py

-# Lint as: python3
 # Copyright 2020 The TensorFlow Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");