Update requirements.txt for OSS keras.
Remove those packages which will be fetched by tensorflow.
-absl-py ~= 0.10
-h5py ~= 2.10.0
-keras_preprocessing ~= 1.1.2
-numpy ~= 1.19.2
+# All the required dependencies should come with tf-nightly package.
+# The rest of the packages are mostly used for testing purpose.
pandas
pydot
scipy ~= 1.5.2
-six ~= 1.15.0
tf-nightly
portpicker
pyyaml
