alsa-mixer: Ignore some elements in the analog-output path

The analog-output path should be suppressed when there are more
specific paths available. Currently that usually doesn't happen. The
suppression can be done with the path subset detection, and this patch
makes that work (another approach would be to mark the elements as
required-absent, like analog-input does, but I like the subset
suppression more, because it requires less stuff in the configuration
files). The problem with listing the now-removed elements in
analog-output.conf was that if the sound card had e.g. a Speaker
element, then the switch behaviour was different between analog-output
and analog-output-speakers, so analog-output was not considered a
subset of analog-output-speakers.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=74609