TFM Cleanup Task

[andreiborodin]

Adding this task to investigate simplifying/cleaning up the TFM overwrites:

• TFM reference in custom C# project: Samples/SimpleNuGetDependency.Unity/Assets/NewtonsoftDependency/NewtonsoftDependency.csproj
• Generated projects both for check-in and otherwise. Today we have it commented.

[andreiborodin]

TFMs in the generated projects is currently commented because it needs to be present in the proj file. We can't even conditionally define it, because we need to import it through import of a props file based on platform/config. This may have a better solution, but leaving that for 1.0.0 release consideration.

Dependencies project has been simplified.