Can't remove U.S. from the languages and input methods menu
It appears that OS X requires a keyboard layout that it recognizes as producing latin characters to be selected as one of the choices in the menu, and it does not recognize ArgenPinYin as one of them. I can deselect the US layout if I select German or Portuguese, for instance, but not if I select Greek or Pinyin - Simplified.

So yes, U.S. can be removed as a language, but some other keyboard layout that OS X recognizes as a Latin layout must first be selected.