message: [fuzzing] remove unused default generate function
  Closes:
diff: @@ -31,12 +31,7 @@ pub trait FuzzTargetImpl: Sync + Send + fmt::Debug {
  /// of the item being generated, starting from 0.
  ///
  /// Returns `Some(bytes)` if a value was generated, or `None` if no value can be generated.
  - fn generate(&self, _idx: usize, _gen: &mut ValueGenerator) -> Option<Vec<u8>> {
  - let mut output = vec![0u8; 4096];
  - let mut rng = rand::thread_rng();
  - rng.fill_bytes(&mut output);
  - Some(output)
  - }
  + fn generate(&self, _idx: usize, _gen: &mut ValueGenerator) -> Option<Vec<u8>>;
  /// Fuzz the target with this data. The fuzzer tests for panics or OOMs with this method.
  fn fuzz(&self, data: &[u8]);
  