Revert "Package ext_gw migration file"

- We no longer plan to package this as part of our custom plugin package
  because we don't want to overwrite core neutron files

This reverts commit 7b8fe8aae4a5a861095bfa36591bb36486b29764.
Merge pull request #79 from davecahill/stable/havana

Revert "Package ext_gw migration file"