avformat/rtpdec.h remove unused variable

Looks like the variable 'cur_timestamp' is not used anywhere.
So remove this variable.
Signed-off-by: Jun Li <junli1026@gmail.com>
Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rtpdec.h

@@ -154,7 +154,6 @@ struct RTPDemuxContext {
     uint16_t seq;
     uint32_t timestamp;
     uint32_t base_timestamp;
-    uint32_t cur_timestamp;
     int64_t  unwrapped_timestamp;
     int64_t  range_start_offset;
     int max_payload_size;