BillingMode: ignore personal OSS subscription

[geropl]

Currently user on a UBP plan are not switching to BillingMode usage-based in case they own a OSS subscription.
We should filter that out, as UBP is strictly more powerful, and should never block anybody from using UBP.

[geropl]

Instead, people default to usage-based in that case.

"Professional Open-Source" is translated to our Usage-Based model separately.

/cc @svenefftinge