Remove checkValidity() and supporting code from Deque.

We've done the same for other more important types (e.g. HashTable) without
complaint.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/23549028

git-svn-id: svn://svn.chromium.org/blink/trunk@157515 bbb929c8-8fbe-4397-9dbb-9b2b20218538