Intent to unship: -moz-outline-radius (now that we follow the border-radius curve for outlines)

[Emilio Cobos Álvarez]

In Firefox 88, Firefox will start following the border-radius in the
outline property. As part of that work, the -moz-outline-radius (a
non-standard property that is not on standards track) will/did stop
doing anything, so I think we should try to unship it.

I'm doing this work on bug 1705509.

The biggest risk here is that some page uses it to feature-detect
Firefox. There's some non-zero usage of this property on telemetry, but:

* Worst case we just lose a cosmetic effect that doesn't work on other
browsers, and...

* If it serves as precedent, literally all the uses of the property I
removed from the codebase while doing this work were not a behavior
change (they were just doing the math that we now do by default to make
outline follow border-radius). So I think usage on the wild will look
fairly similar.

In any case, the property does nothing now, so I think this is a no
brainer unless we find some unfortunate feature detection on the wild.
Let me know if you have any concern.

Cheers,

-- Emilio