Various clean up in wtf/Atomics

Cleanups include:
  - Deleting Atomics.cpp (not included in wtf.gypi)
  - Removing WTF_USE_LOCKFREE_THREADSAFEREFCOUNTED, which is always true
    (as proof, atomicIncrement is used unguarded in IndexedDB and WebAudio).
  - Removing unneeded includes of Atomics.h
  - Remove comment/license block referring to code that no longer exists

R=abarth

Review URL: https://chromiumcodereview.appspot.com/19010007

git-svn-id: svn://svn.chromium.org/blink/trunk@154089 bbb929c8-8fbe-4397-9dbb-9b2b20218538