xen/netback: do some code cleanup

[ Upstream commit 5834e72e

 ]

Remove some unused macros and functions, make local functions static.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20220608043726.9380-1-jgross@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 74e7e1ef

 ("xen/netback: don't call kfree_skb() with interrupts disabled")
Signed-off-by: Sasha Levin <sashal@kernel.org>