Get rid of Python script

[mikesperber]

The presence of the Python script complicates CI as it requires having Python installed, typically another copy of TensorFlow, with the associated library-path issues etc.

It would be nice if this functionality (i.e. the call to the relevant C++ functionality) could be moved to Haskell.

[tomsmeding]

The particular API used by the script is TFLiteConverter, which seems to be actually implemented in Python, and there doesn't seem to be a C++ reimplementation.

Making converter.py use Tensorflow copy in the submodules would be possible, but would require building more of Tensorflow than we currently do; TFLite can be built using cmake but the rest requires Bazel, which requires Java.

On the other hand, converter.py currently just uses the system-installed Tensorflow (absent specific measures to prevent this), which need not even be the same version as the embedded one. Fixing this might by itself be a reason to swallow the Java dependency.

[tomsmeding]

The latest state: converter.py must remain a Python script, because the relevant API is only available in Python (as shown in my comment above). However, we do now build a full Tensorflow copy from the embedded TF submodule, including the Python wheel, and register that in a virtualenv that converter.py can then use. Hence we do not need any system-installed TF any more.