Remove define at include/webvtt/util.h:176

https://github.com/mozilla/webvtt/blob/dev/include/webvtt/util.h#L176 shadows the enum value at https://github.com/mozilla/webvtt/blob/dev/include/webvtt/util.h#L147

```C
  enum
  webvtt_status_t {
    WEBVTT_SUCCESS = 0,
    /* ... */
  };
  /* ... */
#define WEBVTT_SUCCESS(status) ( (status) == WEBVTT_SUCCESS )
```

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