Revert "fixes https://github.com/torchbox/wagtail/issues/564"

This reverts commit 2d5752b6ea3b3b77d4a006d1395532a1c1414dc4.
Revert "add swappable image model migration dependency"

This reverts commit 2e7b7d496d276fece4511da767f0413e9f88af58.
Revert "adjust migration for the swappable model"

This reverts commit 3d1958971e9697257e09e08d5d7cd059f3e3ed24.