Remove some confused callers of confusingAndOftenMisusedAttached

There's no reason to check confusingAndOftenMisusedAttached before calling
Document::prepareForDestruction.

R=esprehn
BUG=299011

Review URL: https://chromiumcodereview.appspot.com/24970002

git-svn-id: svn://svn.chromium.org/blink/trunk@158519 bbb929c8-8fbe-4397-9dbb-9b2b20218538