Remove compatibility code for generated service accounts

[SaschaSchwarze0]

In Remove -sa suffix from generated service account #885, we renamed the generated service account. We should remove the compatibility code in the next release. It actually only applies for updates from before that change for in-progress buildruns while the update happened. If somebody would go from v0.5.1 or earlier directly to v0.7.0, and has running buildruns, then the service account would not get deleted on buildrun completion, though there is still the owner reference that will clean it up on buildrun deletion.