Revert "make *_get_instance_private const-compliant"
This reverts commit
a3a9664ed202303b899ca55625877542309d1a1f.
Constifying the autogenerated get_instance_private functio makes C++
compilers and GCC with -Wcast-qual warn during compilation of GLib and
projects depending on GLib.
Since using const with GObject instances is not a common coding
practice, it's better to revert than trying to make every sigle GType
function const-safe (and possibly add more compiler warnings in the
process).
https://bugzilla.gnome.org/show_bug.cgi?id=745068