[FIXED HUDSON-6025] simplify form.onsubmit handling with combobox.

1) in onblur handler, simply restore old form.onsubmit rather than adding
   another wrapper to call the old handler (which can result in a deep call
   stack if several comboboxes are used prior to form submission).
2) in onsubmit handler when focused, only call the original onsubmit when
   the dropdown is not showing (ie don't call it when we intercept the submit
   to just select an item from the list and return false)
I didn't figure out exactly why, but the current code could result in the
original form.onsubmit handler not being called on form submission when
multiple comboboxes are used.  This simplification seems to resolve the issue.


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@29149 71c3de6d-444a-0410-be80-ed276b4c234a