[sledge] Simplify Control scheduling due to total join
[sledge] Simplify Control scheduling due to total join

Summary:
When domain join operations are total, the control scheduler does not
need to handle the case where joining states is undefined. This leads
to some simplification, and in particular removed the need to expose a
remove operation for the scheduling queue.

Reviewed By: jvillard

Differential Revision: D27828761

fbshipit-source-id: b8cdd2eb6

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