[CT-2812] Remove create_metric property from end user facing Measure spec

[QMalcolm]

Acceptance Criteria

• create_metric property is removed from the UnparsedMeasure definition

Context

Currently we have create_metric property defined on the UnparsedMeasure for UnparsedSemanticModels. The create_metric property is incredibly useful, if it were hooked up. However, we haven't hooked it up, and don't have time to do so before the 1.6.0rc release of core. Thus, for 1.6.x of core we shouldn't allow users to specify this property.

[jtcohen6]

@QMalcolm Is this something we need to do before cutting the v1.6 RC on Thursday? The alternative seems like: leave it in place for now, knowing that we're planning to add in support for this in v1.7

[QMalcolm]

@jtcohen6 I think we don't want to allow users to specify it. It can be something that remains on the graph node, but should be removed from the unparsed version of the node. It's a two line change