Do not use DEFAULT_MAX_THREADS when saving AVIF images
At the moment, DEFAULT_MAX_THREADS may be used when saving an AVIF image.
https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/AvifImagePlugin.py#L57-L59
https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/AvifImagePlugin.py#L167
However, I don't think that's indicated by comments. https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/AvifImagePlugin.py#L16-L20
And it's not mentioned in the docs - https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#avif
max_threads Limit the number of active threads used. By default, there is no limit. If the aom codec is used, there is a maximum of 64.
So this PR suggests not using it when saving. The user can specify it through im.save(max_threads=...).
cc @fdintino