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
They might be a reminiscent of too much copy/paste when factoring cef-binary NuGets out between 3.29 and 31.0.0