[applet] Remove "Show inactive devices" feature

There may be a use case for showing and interacting with inactive devices, but probably not from the applet, which is supposed to be a quick way to interact with the sound system. This commit makes the applet always filter out inactive devices and only show the active ones. The KCM retains the ability to optionally show inactive ones.

Here's what the applet will look like, should this MR be merged:

cc @teams/vdg