Remove a comment which is no longer valid. NFC.

TBR=danakj@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/1115693002

Cr-Commit-Position: refs/heads/master@{#327505}

base/atomic_ref_count.h

@@ -4,9 +4,6 @@
 
 // This is a low level implementation of atomic semantics for reference
 // counting.  Please use base/memory/ref_counted.h directly instead.
-//
-// The implementation includes annotations to avoid some false positives
-// when using data race detection tools.
 
 #ifndef BASE_ATOMIC_REF_COUNT_H_
 #define BASE_ATOMIC_REF_COUNT_H_