fixup: LSPSUrl -> Only support HTTPS, remove scheme/public checks

- Remove scheme field and all related logic from LSPSUrl, only HTTPS is supported by LSPS5.
- Remove is_https and is_public methods; public check is not required by spec.
- parse() now returns LSPS5ProtocolError directly and only accepts HTTPS URLs.
- Simplify and update tests for new constraints.

Author: martinsaposnic