Avoid duplicating channel names as property in neo base extractor when `all_annotations=True`

[h-mayorquin]

As in the title.

There is always a channel_names annotation in neo that we can load with all_annotations=True:

https://github.com/h-mayorquin/python-neo/blob/223e22edb917d8ad5ef8aff4fe7197afc414d5f6/neo/rawio/baserawio.py#L282-L293

This PR modifieds the code so when all_annotations=True the property is not duplicated.

Plus, I am changing the name of the property to channel_names because that's how it is neo and I think is more consistent overall. Another option is to instead change the property name to "channel_name" when loaded from neo annotations so we don't duplicate. I think that the first approach is better.