Filter apphost rather than trying to have nuget suppress it (dotnet/corefx#34785)

NuGet wasn't honoring the Exclude=all across runtime dependencies
but I can make this simpler by just telling depproj filtering
target to remove the file.

Commit migrated from https://github.com/dotnet/corefx/commit/0f6c2fc52b969309308faf442fc77ef99f370344