MAGN-5054 Nodes appear as empty boxes on loading Dynamo

This pull request attempts to resolve an issue where a conflicting version of System.Windows.Interactivity, already loaded in the user's app domain, conflicts with that required by Dynamo. Dynamo's version of System.Windows.Interactivity matches with Revit's. By removing our explicit preload of this library, we fall back to loading Revit's version of the same as that version lives in the Revit application folder which is higher up the assembly resolution path. Testing of Dynamo after removing this preload reveals no problems in interactivity including the user of EventTriggers which are the one object for which the System.Windows.Interactivity library is required. 

For users who were having the reported issue, dropping our preload step is effectively the same as our recommendation to "hide" that dll, as that also falls back to the version loaded using the typical assembly resolution paths.

https://github.com/DynamoDS/Dynamo/issues/2523#issuecomment-59819987

PTAL:
- [ ] @pboyer 

Requires Merge:
- [ ] Revit2015
It doesn't really matter, but you should probably fix this before merging.