placesManager.js: Go back to having isRemovable() call can_unmount().

There isn't enough consistency between various removable device types
and filesystems to be able to rely on can_eject() here.

ref:
#13370

Author: mtwebster

js/ui/placesManager.js

@@ -105,7 +105,7 @@ PlaceDeviceInfo.prototype = {
     },
 
     isRemovable: function() {
-        return this._mount.can_eject();
+        return this._mount.can_unmount();
     },
 
     remove: function() {