tools/ocaml: Delete the Xenctrl.with_intf wrapper

tools/ocaml: Delete the Xenctrl.with_intf wrapper

This wrapper hides an opening and closing of the xenctrl handle, which amongst
other things opens and closes multiple device files.

A process should create one handle at the start of day and reuse that; indeed
there is no guarentee that the process will retain sufficient permissions to
re-open /dev/xen/privcmd at a later point.

With the final user of Xenctrl.with_intf removed, drop the wrapper entirely.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>