remove source from map properly, symmetric to addition.

[jjimenezshaw]

wms.Layer.onAdd is adding this._source to this._map, but is not removed symmetrically on onRemove.

The consequence is that the attribution is never removed if the layer is removed/managed using L.Control.Layers. If you change to different layers that use wms.Layer, the attributions are appended (never removed), even if you remove the layer.