Add additional matrix split to nuspec

Update targets file to only import if matching $Platform
When building cef.redist nuget packages update cef.redist.targets to include a unique target name for 32/64bit otherwise they don't seem to coexist happily
Remove item group entries, don't believe it's relevant anymore
I couldn't find where these were used. If they're required, I'd like to move them to their relevant project.