truemotion2: remove unreachable code

Fixes CID610345.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/truemotion2.c

@@ -222,8 +222,6 @@ static inline int tm2_read_header(TM2Context *ctx, const uint8_t *buf)
         av_log (ctx->avctx, AV_LOG_ERROR, "Not a TM2 header: 0x%08X\n", magic);
         return -1;
     }
-
-    return buf - obuf;
 }
 
 static int tm2_read_deltas(TM2Context *ctx, int stream_id) {