flvmux: aggregate should not push EOS itself

Instead it is expected to return GST_FLOW_EOS, and let the
base class handle that.
diff --git a/gst/flv/gstflvmux.c b/gst/flv/gstflvmux.c
index 68e6572..6b248e4 100644
--- a/gst/flv/gstflvmux.c
+++ b/gst/flv/gstflvmux.c
@@ -1730,7 +1730,7 @@
     if (gst_flv_mux_are_all_pads_eos (mux)) {
       gst_flv_mux_write_eos (mux);
       gst_flv_mux_rewrite_header (mux);
-      gst_pad_push_event (mux->srcpad, gst_event_new_eos ());
+      return GST_FLOW_EOS;
     }
     return GST_FLOW_OK;
   }