Function Refactoring wip
1. Remove CallProvider and all its implementations
2. Remove FunctionDefinition and its references in CallProvider implementations
3. In ExpressionStringBuilder.visitFunctionCall stop considering isOperator case which is not valid anymore
4. Comment out visitFunctionCall implementation in ExpressionValidator, AggregateChecker, ConstantExpressionIdentifier and SimpleEvaluationVisitor. Will revisit when new expression for Drill and Hive functions are added.
5. Remove FunctionInstance class as it is not used anywhere in the code