Make wxWebSessionFactory private

Having wxWebSessionFactory part of the public API implies keeping
compatibility with the possible ways of implementing it which is too
restrictive for no good reason, so move this class to the private header
and don't document it nor wxWebSession::RegisterFactory() (which is now
private).

interface/wx/webrequest.h

@@ -507,17 +507,6 @@ public:
     */
     static wxWebSession* New(const wxString& backend = wxWebSessionBackendDefault);
 
-    /**
-        Allows the registering of new backend for wxWebSession.
-
-        backend can be used as an argument to New().
-
-        @param backend The name for the new backend to be registered under
-        @param factory A shared pointer to the factory which creates the appropriate backend.
-    */
-    static void RegisterFactory(const wxString& backend,
-                                const wxSharedPtr<wxWebSessionFactory>& factory);
-
     /**
         Allows to check if the specified backend is available at runtime.
 
@@ -527,29 +516,6 @@ public:
     static bool IsBackendAvailable(const wxString& backend);
 };
 
-/**
-    @class wxWebSessionFactory
-
-    An abstract factory class for creation wxWebSession backends.
-
-    Each implementation of wxWebSession should have its own factory.
-
-    @since 3.1.5
-
-    @library{wxnet}
-    @category{net}
-
-    @see wxWebSession
-*/
-class wxWebSessionFactory
-{
-public:
-    /**
-        Creates a new web session object.
-    */
-    virtual wxWebSession* Create();
-};
-
 /**
     @class wxWebRequestEvent