Pipeline deregistration

[MaxiBoether]

After #111, we don't offer any gRPC functionality to deregister a pipeline at the selector. We should offer that function and think about what we do with existing data in the database.

[vGsteiger]

I will keep this in mind while designing #113.

[MaxiBoether]

Can be addressed as part of creating the supervisor server that @jenny011 is working on.