[entropy.db] remove support for deprecated provide metadata
[entropy.db] drop support for deprecated eclasses and messages metadata
[entropy.db.sqlite] drop retrieveProvide and searchProvidedVirtualPackage

They are no longer needed since compatibility with the old metadata
has been dropped.
[entropy.db.sqlite] drop retrieveSignatures

This is no longer needed here since support for the old schema
has been dropped (the schema update code will update the tables
anyway)