Remove Scheduler library

The Scheduler library is dependent on Quartz which is capable of persisted schedules. However, after trying several times to integrate that mechanism into Entity Stores of Zest, it is apparent that Quartz assumes an SQL store under the hood, and effectively has leaky abstractions.

On top of that, the Scheduler Library are loosing schedule executions due to its wrong way of integrating persistence with Quartz. This is a very serious bug, and we can't seem to be able to fix it.

We have therefor decided to drop this library entirely.

We recommend that an external scheduler is used instead, possibly via a Rest interface.