Remove deprecated `welcomePageUrl` config option

It's been undocumented since at least the Riot days, and is improperly named. It is replaced by `embedded_pages.home_url` (after https://github.com/matrix-org/matrix-react-sdk/pull/8062 happens).

This issue exists to serve as a thing to search for in code, and a note that we should form a proper deprecation notice to clean it up.