Bug 1730712: Use local vendored `certifi` instead of WPT's r=ahal

Both of them are version `2018.4.16`, but our local one is vendored in a
`pip`-compatible way (it includes a `.dist-info` directory).

FWIW, we'll probably need to keep these two versions in-sync (CI
should already be verifying this in the virtualenv-compatibility
tests) because:
* `certifi` is needed by `sentry-sdk`, a Mach-global dependency
* Web platform tests should use the version of `certifi` that
  exists in the upstream `certifi` repo. These tests may operate in
  a context that includes Mach and its dependencies.

Differential Revision: https://phabricator.services.mozilla.com/D126282

build/common_virtualenv_packages.txt

@@ -44,7 +44,6 @@ pth:testing/mozbase/mozversion
 pth:testing/raptor
 pth:testing/talos
 pth:testing/web-platform
-vendored:testing/web-platform/tests/tools/third_party/certifi
 vendored:testing/web-platform/tests/tools/third_party/h2
 vendored:testing/web-platform/tests/tools/third_party/hpack
 vendored:testing/web-platform/tests/tools/third_party/html5lib
@@ -61,6 +60,7 @@ vendored:third_party/python/atomicwrites
 vendored:third_party/python/attrs
 vendored:third_party/python/blessings
 vendored:third_party/python/cbor2
+vendored:third_party/python/certifi
 vendored:third_party/python/chardet
 vendored:third_party/python/Click
 vendored:third_party/python/compare_locales