Remove MySQL support

[koppor]

At Oracle (refs #6013) and Postgres (refs #4461), updates from other users come in automatically to the local database: A pub-sub mechanism is offered by that databases.

At MySQL, that is not available. This is "OK" since in a "normal" architecture, this is done by a messaging system (https://ultimate-comparisons.github.io/ultimate-message-broker-comparison/ ). - Nevertheless, the user experience is not that good. It is "OK" for a single user setup as no updates from other users come in at a single user setting. However, if a second user joins, they have to manually (!) pull remote changes.

We implemented a setting that for each write at MySQL, the changes are pulled from the server. Thus, if both users constantly change the database, both JabRef instances are in synch.

Moreover, MySQL seems to be slow: #3736

Nevertheless, maintenance of MySQL code costs time. (JabRef code, SQL code, testing MySQL).

[koppor]

MySQL is still the leading platform offered by the Cloud providers.

We need to add Telemetry on that. --> Future Work.