admin: Drop 'internal.h' include from libvirt-admin.h

This is a public library, it shouldn't include anything that is
internal. Including the library in it's current state to an example
application fails the preprocessor phase.

include/libvirt/libvirt-admin.h

@@ -26,8 +26,6 @@
 #ifndef __VIR_ADMIN_H__
 # define __VIR_ADMIN_H__
 
-# include "internal.h"
-
 # ifdef __cplusplus
 extern "C" {
 # endif