xen-balloon: clean up unused functions

Remove some unused functions:

balloon_update_driver_allowance
balloon_release_driver_page
	only used on the (obsolete, removed) flip path in netfront
alloc_empty_pages_and_pagevec
free_empty_pages_and_pagevec
	only used in backend drivers; can be reintroduced when needed

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>