Add method 'removeLayer' to map object for MapScript-PHP

[armin]

MapScript-PHP does not have the method 'removeLayer' for the map object as all the SWIG-based MapScript versions do.

The ability to remove layers from the map object is convenient for working with dynamic layers and modifying the map object in the context of WMS (e.g. hiding or grouping layers in the getCapabilities output).

Setting the layer status to MS_DELETE is in most cases not working as workaround.

[dmorissette]

Duplicate. See ticket #762.