[modules/disk] Remove "custom" instance callback

This callback just makes it really hard to see for the user what is
going on. The alias the user provides should *always* match the
instance, so that it can be used in a meaningful way in the click
action, for example (otherwise, something like "nautilus {instance}"
gets really hard to interpret.

bumblebee/modules/disk.py

@@ -31,9 +31,6 @@ class Module(bumblebee.module.Module):
             bumblebee.util.bytefmt(self._size), self._perc)
         )
 
-    def instance(self, widget=None):
-        return self._path
-
     def warning(self, widget):
         return self._perc > self._config.parameter("warning", 80)