Remove upgrade code from DeploymentConfig migration

WHAT
During the migration from DeploymentConfigs to Deployments, we included code needed to facilitate the upgrade. Once 3scale 2.15 is released, this code can be removed as we don't want to carry it in 3scale 2.16.

HOW
Look through the changes introduced in THREESCALE-10163 (you can use the PR that merged the feature branch as a guide) and remove any changes that were only needed to facilitate the upgrade, i.e. helper functions, upgrade file, DeploymentConfig refs in unit tests, etc.

Note: Most, if not all, of the code that needs to be removed should have the comment

// 3scale 2.14 -> 2.15

before it so you can search for that phrase across the entire project.

DONE

• Any upgrade code needed to facilitate the migration from DeploymentConfigs to Deployments is removed