Drop pygobject_construct() from public API.

These functions were introduced in 2005 because python objects could not
"just" be instantiated using g_object_new(), but this is not true
anymore since the introduction of new-style constructors. Hence this API
has no reason to be there anymore.

Nowadays, people who want to construct GObjects defined in python should
just use g_object_new().

https://bugzilla.gnome.org/show_bug.cgi?id=657814