Remove polaris clean from check polaris commands

[altheaden]

check_env() fails when trying to run polaris clean because it's a deprecated command. I just removed this call from the check.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[altheaden]

Testing

I ran this to create an environment with --check and it worked, where it had been previously failing.

[xylar]

Yep, we don't have that command!