Remove all Revit components from Dynamo repository

## Purpose

This pull request removes all Revit-specific functionality from the main Dynamo repository.  This makes the footprint for the repository far smaller, prevents unsafe host-specific dependencies in core, and generally makes development easier.  

All Revit specific components for Dynamo have been moved here:  https://github.com/DynamoDS/DynamoRevit  

## Note 

This pull request replaces this one, which had some merge failures:
https://github.com/DynamoDS/Dynamo/pull/3377