status: Stop using .panel-status-button

status: Stop using .panel-status-button

This used to be the style-class for status icons (i.e. icon-only
top bar items). It got unused a while ago when the class used
by status icons stopped using it - except for the keyboard indicator,
which set the class manually to appear as status icon despite not
being a real icon.
Now that the button highlight is provided by the .panel-button class
on a parent, the obsolete class results in a double border on the
keyboard indicator when active - just drop it from there as well
to fix.