Remove the QSQLITE3 driver

Initially, this fork of the upstream QSQLITE driver has been introduced
because the upstream driver was lacking some features needed for proper
concurrency and multi-threading support. However, those features only
really make sense when using the Shared Cache mode, which has been
deprecated for years and we have moved away from it recently in favor of
the modern WAL mode. With the WAL mode, this fork has no real benefit
over the upstream Qt driver (other than being outdated). Therefore we
can remove it and use the upstream one everywhere.