env: delete old unused environments

Clang is installed from CIPD now, the debug VMs aren't in use anywhere
AFAICT and we already have a lot of the debug tooling installed, and
stretch has had its security updates discontinued since 2020.

Change-Id: Ie9cf0e8e8bd37929047443131f9ada5c34552da7
Reviewed-on: https://go-review.googlesource.com/c/build/+/672095
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mknyszek