Add rake task to remove unused images
Add README note about rake prune_images task
Remove unused images
Revert "Add README note about rake prune_images task"

This reverts commit 9c123befe20af1418f4f786bfae8b63daddbcdf4.
Revert 9c123be..3293f05

This rolls back to commit 9c123befe20af1418f4f786bfae8b63daddbcdf4.