[Documentation]: Remove example in ophys tutorial for TwoPhotonSeries external file option

[rly]

What would you like changed or added to the documentation and why?

The ophys tutorial https://pynwb.readthedocs.io/en/stable/tutorials/domain/ophys.html shows two ways of storing OnePhotonSeries and TwoPhotonSeries data: internally and externally. External file storage of neurophysiological data goes against NWB best practices. Users also cannot upload such data to DANDI.

We should remove this option from the tutorial.

Do you have any interest in helping write or edit the documentation?

Yes.

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