core: Remove leftover getAuthUserName and SQL

core: Remove leftover getAuthUserName and SQL

Remove Storage::getAuthUserName(), including PostgreSQL and SQLite.
Remove the corresponding select_authusername.sql files, too.

This avoids unused code and may avoid confusion in the future.  Any
time a username-for-ID is needed, Core::_authUserNames should be
consulted instead.

NOTE: Before using Core::_authUserNames, public functions should be
created and caching should always be done on core startup, instead of
only when --strict-ident mode is enabled.