Remove deprecated date_histo intervals

[polyfractal]

We split date_histo intervals between calendar and fixed intervals in #33727. At the same time, we deprecated the older, unified interval parameter.

It's been deprecated since 7.2, so we should be good to remove from 8.0 in my opinion. There is a non-negligible amount of code to handle the old intervals and it'd be great to clean that up.

This issue is a placeholder so we don't forget to remove it as 8.0 grows nearer :)

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)

[not-napoleon]

I resolved this with #75000, I just forgot to tell github to close this one.