Amgen Inc. will shed roughly 300 of 750 Onyx Pharmaceuticals employees and shutter the South San Francisco facility the giant biotech picked up in its $10 billion acquisition of Onyx less than 18 months ago. The move, following positive news last week on the signature drug of the Amgen-Onyx deal, is the proverbial shoe-drop that Bay Area biotech followers long expected as Thousand Oaks-based Amgen undertakes another restructuring, this time of its cancer drug business.