Intent to unship: -moz-user-modify

[Emilio Cobos Álvarez]

-moz-user-modify is a prefixed CSS property which does _almost_ nothing.

It has some effect on caret painting (as in, you could make a
contenteditable element not paint the caret), but there are better ways
of doing that (caret-color: transparent or so).

It is also documented to do nothing[1].

Given that, I plan to try to remove it. Only real risk is that someone
uses for feature detection or so, so I expect the risk to be pretty minimal.

Let me know if there's any concerns,

-- Emilio

[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/user-modify

[Emilio Cobos Álvarez]

Forgot to say, this work is happening in bug 1920118.