drop migrateImage call
The migrateImage call allows migrating pre-1.8 image builds from the old
model (no build entry) to the new model (image build type). This call
was added in 1.8 (4.5 years ago) as a one-time tool for migration. It
is only available if the EnableImageMigration is set on the hub.
See also: https://docs.pagure.org/koji/migrating_to_1.8/
Related: https://pagure.io/koji/issue/612