Remove docs about deprecated pypi-mirror tool

The pypi-mirror tool is no longer supported by the infra team, or
reccomended for installs, and is known to have various bugs that will
not be fixed.

Remove mentions of it from the docs and point people at devpi-server
instead.

Change-Id: Ieedb8bf9bee2f5932edac6b9747e645d0f4eb08c
Merge "Remove docs about deprecated pypi-mirror tool"