Revert "Issue #2711353 by rakesh.gectcr, Jo Fitzgerald, quietone, chx, Yogesh Pawar, Pavan B S, biguzis, phenaproxima, mikeryan, vasi, iMiksu, Berdir, dawehner, benjy: Migrate never unsets existing data for content entitites"

This reverts commit 3d524aaa.

Issue #2711353 by rakesh.gectcr, Jo Fitzgerald, quietone, chx, Yogesh Pawar, Pavan B S, biguzis, phenaproxima, mikeryan, vasi, iMiksu, Berdir, dawehner, benjy: Migrate never unsets existing data for content entitites

Issue #2859246 by Jo Fitzgerald, phenaproxima, alexpott, xjm: Allow MigrateExecutable to be constructed without a message object

Issue #2783715 by claudiu.cristea, chx, mikeryan, heddn, phenaproxima: Entity destination ID schema should match entity ID definition

Revert "Issue #2783715 by claudiu.cristea, chx, mikeryan, heddn, phenaproxima: Entity destination ID schema should match entity ID definition"

This reverts commit 3348de12.

Issue #2783715 by claudiu.cristea, chx, mikeryan, heddn, phenaproxima: Entity destination ID schema should match entity ID definition

Issue #2225775 by vasi, quietone, phenaproxima, chx, penyaskito, mikeryan, esclapes, vprocessor, steinmb, Gábor Hojtsy, Marc Angles, bwinett: Migrate Drupal 6 core node translation to Drupal 8