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

The active input and output streams aren't added through add_stream()
so they weren't being mapped to their respective volume bars,
resulting in updates to the volume through other means not being
reflected in our UI.

Moving the mapping to bar_set_stream() which is called for every
stream fixes the problem. Also, there's no value in having a
helper function to do a g_hash_table_insert() and not doing the same
for the respective g_hash_table_remove()s so remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=738869