rtpL8pay: don't try to modify a read-only structure

Just remove the code. It's not doing anything useful anyways. The modified
caps are the result of a caps query, so either not used afterwards of a
reference to some internal caps of another element that should not be
modified.

https://bugzilla.gnome.org/show_bug.cgi?id=796837
diff --git a/gst/rtp/gstrtpL8pay.c b/gst/rtp/gstrtpL8pay.c
index 86e7b22..cf2a3b9 100644
--- a/gst/rtp/gstrtpL8pay.c
+++ b/gst/rtp/gstrtpL8pay.c
@@ -196,16 +196,10 @@
 
       if (gst_structure_get_int (structure, "channels", &channels)) {
         gst_caps_set_simple (caps, "channels", G_TYPE_INT, channels, NULL);
-      } else {
-        /* Support any number of channels, if not explicitly specified */
-        gst_structure_remove_field (structure, "channels");
       }
 
       if (gst_structure_get_int (structure, "clock-rate", &rate)) {
         gst_caps_set_simple (caps, "rate", G_TYPE_INT, rate, NULL);
-      } else {
-        /* Support any rate, if not explicitly specified */
-        gst_structure_remove_field (structure, "rate");
       }
 
     }