remove gnttab specific member from struct file

remove gnttab specific member from struct file

The gnttab specific union member in struct file is no longer
needed, so remove it.

The gnttab file type and its associated handling can be removed, too,
as libxengnttab is now supplying a struct file_ops via a call of
alloc_file_type().

This removes all contents of CONFIG_LIBXENGNTTAB guarded sections, so
this config option can be removed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>