build_image: drop support for python2 in release images

These symlinks are only for dev_install to install Python 2 later on.
They aren't used by test images.  Drop support for Python 2 there.

BUG=chromium:1007152
TEST=CQ passes

Change-Id: If155058fae29519c41803be02eb9df812d61dac9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2726547
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed