Remove disk image transfer type steps

These were pulled in by accident when cherry-picking unrelated
taxonomy changes. Now that disk image behaviour can be FPR-drive, there
is no need to handle this in the database - it can be handled purely
via FPR rules.

There is still a Disk Image transfer type but it exists solely for
metadata entry. It otherwise operates exactly like the Standard
transfer type.
Component metadata: use textarea for image fixity