Remove now-unnecessary VectorTraits specializations.

Following r183947's improvements to VectorTraits accuracy, remove some
redundant specializations.

R=tkent
BUG=

Review URL: https://codereview.chromium.org/654933004

git-svn-id: svn://svn.chromium.org/blink/trunk@184046 bbb929c8-8fbe-4397-9dbb-9b2b20218538