[ARM] Remove isThumb() as it's not used and it's not in the right place.

Architecture specific code should reside in architecture specific directory
not in Atom. Looks like there are no efforts being made at this moment to
support ARM, so let's remove it for now.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D959