Issue #2541580 by mikeryan: Remove obsoleted functionality from core Migrate

core/modules/migrate/src/MigrateExecutable.php

core/modules/migrate/src/MigrateExecutableInterface.php

...odules/migrate/src/Plugin/MigrateDestinationInterface.php

...igrate/src/Plugin/migrate/destination/DestinationBase.php

...es/migrate/src/Plugin/migrate/source/SourcePluginBase.php

core/modules/migrate/src/Plugin/migrate/source/SqlBase.php

...te/tests/src/Unit/MigrateExecutableMemoryExceededTest.php

.../modules/migrate/tests/src/Unit/MigrateExecutableTest.php

.../modules/migrate/tests/src/Unit/TestMigrateExecutable.php