Remove DKS

@adelmann @gsell and I propose to remove DKS for the following reasons:

1. Maintaining DKS is costing valuable manpower resources
2. There are several open DKS issues, especially many regression tests are broken (#405 (closed))
3. On the users meeting Andreas presented a long-term possible generic replacement with Kokkos.

Todo List:

• remove from src (!353 (merged))
• update wiki
• remove from Manual (wasn't mentioned)
• remove from regression-tests (regression-tests!16 (merged))
• remove from NightlyBuild (NightlyBuild!13 (merged))