When running MSBuild targets the TargetFramework property was explicitly
set so that multi-target framework projects work for targets such as
ResolveAssemblyReferences. However setting this property prevents
MSBuild from building all frameworks for a project using the
cross-target MSBuild targets. To fix this when the Build or Clean
MSBuild targets are run the TargetFramework is not set. This allows
all frameworks to be built by MSBuild.

Fixes VSTS #572330 - Support building multi-targeted projects