Remove last_size check.

 * The variable last_size is not used in any part of the code other than
   this two lines. This seems to have been added for debug purposes and
   was left there.

 * This code starts triggering on a kernel >= 3.14 (Ubuntu 14.04 uses
   3.13, so this went unnoticed). It causes a lot of flood because it
   triggers constantly.

 * Example of the output from snabswitch when running same_vlan.ports
   with both VMs running Kernel 3.14.23 and running an iperf test
   (MTU 1500) between both VMs:
```
      size=  2036
      size=  1524
      size=  2036
      size=  1524
      size=  2036
      size=  1524
      [ .... repeated ad-infinitum ]
```
 * I have bisected the Linux kernel, the commit that causes this print to
   trigger is fb51879dbceab9c40a39018d5322451691909e15 (merged on 3.14)
   https://git.kernel.org/linus/fb51879