context: Remove RuntimeData and gjs_runtime_init/gjs_runtime_destroy

Now that RuntimeData only contains one pointer, we can just swap
out the allocation with a direct reference to the pointer instead.
The leftover initialization functions, not needing to do memory management,
become one-liners and we can simply squash them where used.

https://bugzilla.gnome.org/show_bug.cgi?id=691109