This reverts commit 52f23db74a.
Constifying these macros make 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