Removed repeated conditionnal statements

The exact same conditionnal statement throwing error if ptype is unknown
in Plugin.register() is already executed by
MetaPlugType.type_from_name(), which is called right after it. Guessing
which class should be concerned by such a check, I ended up removing it
in Plugin.

lodel/plugin/plugins.py

             raise PluginError("No plugin named %s found" % plugin_name)
         pinfos = pdcache['plugins'][plugin_name]
         ptype = pinfos['type']
-        if ptype not in MetaPlugType.all_ptype_names():
-            raise PluginError("Unknown plugin type '%s'" % ptype)
         pcls = MetaPlugType.type_from_name(ptype)
         plugin = pcls(plugin_name)
         cls._plugin_instances[plugin_name] = plugin