Remove PIP_INSTALL trace

This trace artifact is no longer used in
anvil so instead of keeping it around just
remove it.

Change-Id: I7811de18b314c2c728543fdbb8b9eee47349d69a
Various tweaks to pip downloading

- Move the pip dependency downloading function + logic
  to the pip_helper module instead of being in the base.py
  module (since its a pip utility function)
- Add a retry function to utils and use it instead of the
  custom retry module used for downloading pip dependencies.
- Cleanup other various code in pip_helper.py

Change-Id: I038e4d73c814ea06a18eae61e41c5b7bef779b9d
Merge "Various tweaks to pip downloading"