[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.

Nell’issue non sono indicati file o test. Inizia individuando la configurazione di build di helper-image e i riferimenti a Python 2.7 e pydevd-pycharm; verifica come vengono assemblate le immagini v1 e 2.0. Il lavoro è completato quando l’immagine 2.0 non include più nessuno dei due componenti, mentre la configurazione v1 documentata rimane disponibile per gli utenti che hanno bisogno di Python 2.7.

Scritto dal modello di indicizzazione a partire dal testo della issue.