CA-375277 xenctrlext: remove dead code, clean up (#6881)

This picks up an earlier pull request by @edwintorok that was abandoned.
We need to be sure that the code we are calling it thread safe.

Author: lindig

ocaml/libs/xenctrl-ext/dune

@@ -6,6 +6,5 @@
   (language c)
   (names xenctrlext_stubs)
  )
-  (c_library_flags (-L/lib64 -lxenforeignmemory))
 )