aviobuf: Remove a senseless ifdef in avio_seek

This seemed to assume that one never used writing avio unless
muxers or networking was enabled.

This ifdef is a remnant since 8fa641f8.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/aviobuf.c

@@ -215,12 +215,10 @@ int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
     } else {
         int64_t res;
 
-#if CONFIG_MUXERS || CONFIG_NETWORK
         if (s->write_flag) {
             flush_buffer(s);
             s->must_flush = 1;
         }
-#endif /* CONFIG_MUXERS || CONFIG_NETWORK */
         if (!s->seek)
             return AVERROR(EPIPE);
         if ((res = s->seek(s->opaque, offset, SEEK_SET)) < 0)