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 With the WAL mode, this fork has no real benefit over the upstream Qt driver (other than being outdated).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.

I've been running with the stock QSQLITE driver for several months now with no issues.