Remove .columns

[annie]

(Based on a suggestion from Mike)

Remove .columns from __table. It's no longer needed since we always guarantee that we'll return a .schema, and .schema is a superset of the information in .columns.

One thing I'm not too sure about – if the source has a .columns property and the source isn't modified in __table, the .columns property will pass through and remain in the output. Is that ok, or do we need to standardize __table by making sure .schema is the only property on the output?

[libbey-observable]

These changes look good to me. If a source comes in to __table() with .columns and thus passes through and gets returned with .columns, that seems okay to me, but if we want to dig in deeper on that I'd be up for that, too.

[annie]

yep that makes sense to me! thinking about it a bit more, there could be any number of properties on source that we just pass through right now, so it makes sense to treat .columns as just one of those.