Handle a case where the JSONP request succeeds after the timer has been fired
by checking to see if the script tag has already been removed.

Issu: 3993
Patch by: dtm@google.com
Review by: jat


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6290 8db76d5a-ed1c-0410-87a9-c151d255dfc7