cassini-bsp: Use unpadded image for Corstone-1000 FVP

The new OE inherit_defer feature seems to have broken custom image
types, which we currently use to create a padded wic image for the
Corstone-1000 FVP.

Since the FVP now supports unpadded images, rather than debug the issue
or wait for a fix, re-configure the FVP to use the unpadded wic image.

Changelog: other
Signed-off-by: Adam Johnston <adam.johnston@arm.com>