Bug 1717051: Remove obsolete python module paths r=ahal

The three removed paths don't exist in-repo, and after a cursory glance
they don't appear to be populated dynamically.

Note that the removal of the `six` path for WPT is different: it's
technically just incorrect, and should amended to point to
`$WPT/tools/third_party/six`. However, Python only allows a single
instance of a library to exist in import scope, and we're already
consuming `six` from the Firefox-wide vendored 3rd-party libs.

Differential Revision: https://phabricator.services.mozilla.com/D119825

build/common_virtualenv_packages.txt

@@ -20,8 +20,6 @@ pth:testing/condprofile
 pth:testing/firefox-ui/harness
 pth:testing/marionette/client
 pth:testing/marionette/harness
-pth:testing/marionette/harness/marionette_harness/runner/mixins/browsermob-proxy-py
-pth:testing/marionette/puppeteer/firefox
 pth:testing/mozbase/manifestparser
 pth:testing/mozbase/mozcrash
 pth:testing/mozbase/mozdebug
@@ -46,7 +44,6 @@ pth:testing/mozbase/mozversion
 pth:testing/raptor
 pth:testing/talos
 pth:testing/web-platform
-pth:testing/web-platform/tests/tools/six
 pth:testing/web-platform/tests/tools/third_party/certifi
 pth:testing/web-platform/tests/tools/third_party/h2
 pth:testing/web-platform/tests/tools/third_party/hpack