Remove invalid ASSERT

Remove invalid ASSERT

There is a possibility for it to have been updated already if newDatums
are requested before emittersChanged is reached. This became possible
when emitters stopped having strict particle emission limits.

Change-Id: I45d1564f56547418127446e3138ff21deec891c3
Reviewed-on: http://codereview.qt-project.org/4753
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Martin Jones <martin.jones@nokia.com>