dev-python/virtualenv: Drop package

Nothing uses this anymore.

BUG=b:318531403
TEST=CQ

Disallow-Cq-Looks-For-Green: True
Change-Id: I52b9512234d6404436a6f46cc4405058506d2010
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/5420790
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Allen Webb <allenwebb@google.com>
Auto-Submit: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Jack Rosenthal <jrosenth@chromium.org>