lib: Move internal binding verification API to repo.c

`ostree-repo-pull.c` is rather monstrous; I plan to split it
up a bit.  There's actually already a `pull-private.h` but
that's just for the binding verification API.  I think that one
isn't really pull specific.  Let's move it into the "catchall"
`repo.c`.

src/libostree/ostree-cmdprivate.c

@@ -24,7 +24,6 @@
 #include "ostree-cmdprivate.h"
 #include "ostree-repo-private.h"
 #include "ostree-core-private.h"
-#include "ostree-repo-pull-private.h"
 #include "ostree-repo-static-delta-private.h"
 #include "ostree-sysroot-private.h"
 #include "ostree-bootloader-grub2.h"