Removed migration tests for deprecated nodes

Removed migration test cases for deprecated nodes. 
Removed check for non-null node previews from migration test framework as number nodes do not have previews plus some valid nodes can have null values. The more appropriate check is for tests to not throw any assertions on execution, which is already being tested.

This PR addresses: http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4126

@riteshchandawar PTAL
I don't think it should be a problem. If we want to put it back, we can always add this call later on as the API itself is not being removed.