Remove ShareAlike license clauses from this repo

[towynlin]

I interpret the ShareAlike clauses as not having any impact on product creators' usage of these files in their own products. Specifically, if you're building a product on Particle, you should use these files to make sure your hardware meets specifications. The hardware you thus create does not need to be open source.

However, just to eliminate confusion, @mohitbhoite, please go ahead replace the Attribution-ShareAlike license here with an Attribution license.

The ShareAlike clauses do make sense for most of our other hardware repos, but this one in particular is primarily used by our enterprise customers in the creation of Particle-powered products.

cc: @jenesaisdiq