Remove text-mangling of default values.

I don't seem to get any failures with this locally. Would need a ticket and docs for merge, making this a PR for the CI.

The behaviour is weird at present - if you pass a fixed value then we force to text, but we don't for callable defaults, implying tht we support non-text values elsewhere. I've had some issues to fix or work around in contrib.postgres as a result of this code.