wp: remove WpSession and WpEndpointLink

We have ended up not using them, so let's not carry them
in the ABI of 0.4

We can always revert that, but let's first decide how
these objects should be used

modules/module-lua-scripting/api.lua

@@ -154,6 +154,5 @@ SANDBOX_EXPORT = {
   Node = WpNode_new,
   LocalNode = WpImplNode_new,
   Link = WpLink_new,
-  ImplSession = WpImplSession_new,
   Pod = WpSpaPod,
 }