Revert "xen-netback: Aggregate TX unmap operations"

Revert "xen-netback: Aggregate TX unmap operations"

This reverts commit e9275f5e2df1b2098a8cc405d87b88b9affd73e6. 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>