MAINTAINERS: Drop myself as xen netback maintainer

Wei has been picking this up for quite a while now.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>