avformat/rtpdec: Remove next pointer from Protocol Handlers

Forgotten in 6197453761 (notice that
RTPDynamicProtocolHandler is not a public struct, so one can remove
the linked-list pointer immediately (unlike in most other patches of
this kind)).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/rtpdec.h

@@ -134,8 +134,6 @@ struct RTPDynamicProtocolHandler {
     /** Parse handler for this dynamic packet */
     DynamicPayloadPacketHandlerProc parse_packet;
     int (*need_keyframe)(PayloadContext *context);
-
-    struct RTPDynamicProtocolHandler *next;
 };
 
 typedef struct RTPPacket {