Remove implementor from HiveRelNode

Calcite's VolcanoPlanner [1] relies on calling convention [2]. In Hive this is represented by the HiveRelNode interface's CONVENTION field.

This interface has to be implemented by all Hive operators to have the Hive calling convention behavior. The interface also defines the

 void implement(Implementor implementor);

method but none of the operators gives an implementation and the method is never called.

[1] https://15721.courses.cs.cmu.edu/spring2017/papers/14-optimizer1/graefe-icde1993.pdf
[2] https://arxiv.org/pdf/1802.10233.pdf (Section 4, traits)