remove event channel specific struct file definitions

remove event channel specific struct file definitions

The event channel specific union member in struct file is no longer
needed, so remove it together with the associated structure
definitions.

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

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

Add an extern declaration for event_queue as it is used by
libxenevtchn.

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