Remove some redundant ASSERTs

Node::attach already has this ASSERT.

R=esprehn
BUG=299011

Review URL: https://chromiumcodereview.appspot.com/24967002

git-svn-id: svn://svn.chromium.org/blink/trunk@158429 bbb929c8-8fbe-4397-9dbb-9b2b20218538