plugins/elements/gstmultiqueue.c: Remove previous hack of unsetting the flushing flag for the source pad instead of a...

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c:
(gst_multi_queue_src_activate_push), (gst_single_queue_new):
Remove previous hack of unsetting the flushing flag for the source pad
instead of activating it. Instead, fix the source pad activate function
so that it no longer depends on having a parent set or not.