[Fix doc example] - ProphetNetDecoder
# What does this PR do?

The doc example in `ProphetNetDecoder` will fail

https://github.com/huggingface/transformers/blob/68cc4ccde25b1ed8a9b77fdf6f78c833bdff0e9c/src/transformers/models/prophetnet/modeling_prophetnet.py#L1473-L1474


ProphetNetDecoder never set `is_decoder = True`. It is set only at the level at `ProphetNetModel` and `ProphetNetForCausalLM`.

This PR removes that assertion.



