media-session: don't remove default.configured when devices go away

There's no particular problem to have default.configured to point to a non-existent device, it will be ignored. This was left over when default and configured default were made separate things.

default-nodes also doesn't need to track the id of the configured default, so remove also other stuff that's no longer necessary. It can simply load the values to metadata on startup, and save them to a file when they are changed, without needing to understand what the values mean.

Also fixes commit 70715623 causing defaults to be forgotten when nodes are removed.