remove DatabaseInterface::trackAdded signals to improve performance

Summary:
remove DatabaseInterface::trackAdded signals to improve performance

should help make startup and indexing faster

in preparation for optimisation of init when tracks are already known

Test Plan: tests are OK

Reviewers: Elisa, astippich

Reviewed By: Elisa, astippich

Maniphest Tasks: T7603

Differential Revision: https://phabricator.kde.org/D12609