Revert "xen-netback: Aggregate TX unmap operations"

Revert "xen-netback: Aggregate TX unmap operations"

This reverts commit 3475624a8e4891dc1cd35aa73571a98cdb958d52. This commit is the
last in the netback grant mapping series, and it tries to do more aggressive
aggreagtion of unmap operations. However practical use showed almost no
positive effect, whilst with certain frontends it causes significant performance
regression.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>