Remove check if address is on local subnet for unicast scanning

**Short feature/function description**
The feature will break when in NAT mode (e.g. host network mode in docker). So it's problematic and best to remove for now.

**What needs to be done?**
Revert #581.

**Is this a breaking change?**
Yes, the `NonLocalSubnetError` exception will no longer be used and marked as deprecated

**Anything else worth knowing?**
See #765.
