Intent to unship: -moz-user-input

[Emilio Cobos Álvarez]

-moz-user-input is a not-very-well-defined mozilla-specific property, that causes some elements to behave like disabled form controls.

In general, there are better tools for that nowadays, such as the inert attribute.

Looking at use counters, they are used in ~0.1-0.2% of pages (which is a bit, but not terrible).

A  lot of the usage I can find is for the `disabled` value which was unshipped already in bug 1405087.

This will be done in bug 1935198, behind the pref layout.css.moz-user-input.enabled.

Let me know if there's any concern with proceeding.

 -- Emilio