Remove `MessageFormat.formatToParts`

[stasm]

The main motivation behind formatToParts was the use-case of passing React elements into translations in fluent-react. With a plan to implement overlays in fluent-react (#103) this use-case goes away.

formatToParts has been buggy and under-spec'ed. Let's remove it for good. It will also make the interface of MessageContext very simple: the format method always returns a null or a string.