Revert "sound: Fix the main volume bars to correctly display changes in volume"

This results in crashes when switching inputs or outputs, as other parts
of the code expect only application bars being added to that hashtable.
The bar was destroyed when switching outputs (as the stream is removed before
a new one being added) but with the bar pointing to freed memory.

This reverts commit 22fa7f12.

https://bugzilla.gnome.org/show_bug.cgi?id=746227