Remove the Element.prefix setter

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/bka_hQuzAu0/tjD-GhJ6zDAJ

Attribute::setPrefix() was unused and showed up in grep.

BUG=431552

Review URL: https://codereview.chromium.org/688333002

git-svn-id: svn://svn.chromium.org/blink/trunk@185009 bbb929c8-8fbe-4397-9dbb-9b2b20218538