Stop declaring "Cps" project capability

The CPS project capability is added automatically by the CPS project factory. It never has to be supplied (and should not be) by a project capability provider.
Move declaration of ProjectReferences capability to .targets

IProjectCapabilityProviders should be used sparingly, and this project capability works if defined in the .targets file.
Merge pull request #1 from AArnott/patch-1

Stop declaring "Cps" project capability