cleanup: remove Products.validation

[gforcada]

It is no longer used by Plone core.

See https://github.com/plone/Products.validation/issues/9

I thought of leaving the constrain, but as anyway there are no new releases, the latest one is still safe to use 🍀

[mauritsvanrees]

Since collective.easyform still uses this, I would say: leave it where it is in versions.cfg for the 6.0 series.

In 6.1 we could either move it to versions-ecosystem.cfg, or drop it altogether, especially if we move the relevant parts to collective.easyform.

[jensens]

Easier to start over with a fresh one than rebasing here, see #933