Clarify what code handles 'An end tag whose tag name is "script"'

The "running a script" algorithm was renamed to "prepare a script" in
2011 <http://html5.org/r/5855>, but HTMLScriptRunner::runScript
doesn't actually correspond to that algorithm, so remove the comment.

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/152903004

git-svn-id: svn://svn.chromium.org/blink/trunk@166345 bbb929c8-8fbe-4397-9dbb-9b2b20218538