[2.0] Drop Python 2.7 support and pydevd-pycharm

The python helper image is a monolithic image that includes debugging support for all supported versions of Python. Maintaining older versions becomes a drag on space. For 2.0, we should:

- Drop Python 2.7 support: Python 2.7 was EOL January 1, 2020, and the [only service supporting Python 2.7 on Google Cloud](https://cloud.google.com/python/docs/python2-sunset) is App Engine, which doesn't use these debugging helpers.
- Drop `pydevd-pycharm`: it was added to support Cloud Code for IntelliJ, but they were unable to get it to work, whereas pydevd worked fine.

Users requiring Python 2.7 support can use Skaffold's `debug-helpers-registry` config setting to point to the v1 debug images (`gcr.io/k8s-skaffold/skaffold-debug-support/v1`) their own images.