Intent to unship: mozactionhint attribute

[Mathew Hodson]

Gecko implements a non-standard `mozactionhint` attribute on `<input>` and `<textarea>` elements. It was implemented for Fennec, and it's mentioned here:

* https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#non-standard_attributes

The attribute indicates the appropriate label for the Enter or Return key on a virtual keyboard.

It has since been replaced by the standardized `enterkeyhint`, which shipped in Firefox 94.

* https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/enterkeyhint

The plan is to remove support for `mozactionhint` in bug 1735980, which will ship with Firefox 119.

* https://bugzilla.mozilla.org/show_bug.cgi?id=1735980