Revert "upnp: fire an update source message when a UPnP server becomes available/unavailable"

This reverts commit ff7416411df2d7f83906417232eee9466419be7b.
Revert "media library: add UPnP servers to sources://<mediatype>"

This reverts commit 762602329985513e071da34502a5b4af2ec83f65.
Merge pull request #5639 from Montellese/revert_upnp_sources