Remove workaround for DoctrineMigrationsBundle

[marein]

The bundle uses Doctrine\Migrations\Configuration\Connection\ExistingConnection by default. Although the connection is configurable via a separate config file or cli argument, this will not work because the ExistingConnection behaviour prevents overriding.

This issue is currently not resolvable and should be checked again from time to time.

The workaround was introduced in #64.

[marein]

The problem above is resolved. There's a new issue. To be able to use multiple connections, the bundle requires a default connection. Still not resolvable and should be checked again from time to time.

The new workaround was introduced in #70.