CheckedArithmetic: remove unneeded workaround code

Removed a workaround code for http://llvm.org/bugs/show_bug.cgi?id=10801
which is already fixed.

Review URL: https://codereview.chromium.org/135173005

git-svn-id: svn://svn.chromium.org/blink/trunk@166233 bbb929c8-8fbe-4397-9dbb-9b2b20218538