Make editing/plaintext-only stop doing `.contentEditable = "plaintext-only"
If dom.element.contenteditable.plaintext-only.enabled is false,
the API throws an exception. Therefore, the following tests cannot check
the result. So, we should not use it. Instead,
.setAttribute("contenteditable", "plaintext-only") is fine.
Differential Revision: https://phabricator.services.mozilla.com/D224576
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1922712
gecko-commit: d7c82075e3effb6b62e01a92919ad3c646d5d0c3
gecko-reviewers: m_kato