[SBRP] generate.sh doesn't handle "<icon>" in reference package nuspecs

[dagood]

The tooling generated some nuspecs with <icon>Icon.png</icon>, but without placing the file. It should either point at a central file (avoid many duplicates of the same file) or remove the icon because we don't need it in this context.

[tkapin]

@MichaelSimons - I've added area-product-experience label to this based on our recent conversation. Please feel free to remove it if you think it shouldn't be labeled so.