[css-forms-1] Drop input-security?

[lukewarlow]

Given we've resolved to add a password visibility toggle to password inputs in the base appearance mode (#11845 (comment)) can we now drop input-security from the specification (it's been moved to css-forms from css-ui)

See also #6788 for why an author accessible property is considered a bad idea.

Alternatively, could it be useful to explain the password toggle in terms of this property but not expose it outside the UA stylesheet (similar to overlay?).