Remove `editor` role from the test service account

[tmatsuo]

Currently the service account used for our Kokoro builds has editor role. While this is convenient, strictly saying we should stop using the deprecated role and instead add specific minimum roles to the service account.

[tmatsuo]

Depending on how to tackle this, I would consider this to be blocked by #3821

What I'll try is to remove the role, and run periodic builds to find tests which needs additional permissions/roles.