Remove duplicate alert roles in moz-message-bar

The change from upstream in #45061 (closed) has meant the moz-message-bar sets two elements with the role="alert" attribute.

NOTE: The original issue in #43072 (closed) is no longer a problem since at least Orca 47. In particular, the message bar content is now announced without any accessible name or description. However, as discussed in bugzilla bug 1895857, an explicit accessible name and description is still useful to give context. Especially in non-chrome contexts where we rely on our own alerts.