Remove unused echoPrivate.checkAllowReddemOffers function

BUG=NONE
TEST=browser_tests:*Echo*

Review URL: https://codereview.chromium.org/23470007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226905 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226905