Note by-name usage
Remove a number of unreferenced and self-referencing only classes
Remove actually unused TScalar#isCandidate() and TCommutativeOverloads.

isCandidate() had no implementations other than return null, which
implied "true" during overload resolution. This was an added only to
support funny, type-dependent function overloads that is no longer used.

TCommutativeOverloads were just an artifact of an abandoned feature that
no longer had any implementations.
merge nathanlws: Remove miscelaneous unreferenced

Merge pull request #1591 from nathanlws/sql-layer/remove-misc-unreferenced
https://github.com/FoundationDB/sql-layer/pull/1591