Remove needless comments from ScriptPromise

This is the last review comment at http://crrev.com/26004002 ,
but I didn't notice it.

BUG=304154
R=haraken

Review URL: https://codereview.chromium.org/26552008

git-svn-id: svn://svn.chromium.org/blink/trunk@159780 bbb929c8-8fbe-4397-9dbb-9b2b20218538