aggregator: Reset the pad's first buffer flag with the rest
aggregator: Reset the pad's first buffer flag with the rest

There is not reason to have separate code to reset this one.

https://bugzilla.gnome.org/show_bug.cgi?id=781673

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer