util: storagefile: Drop image format probing by file suffix

util: storagefile: Drop image format probing by file suffix

Probing by file suffix was meant to be a last resort if probing by
contents fails or is not supported. For most formats we never specified
any suffix. There's a few formats implementing both magic bytes and
suffix and finally DMG which had only suffix probing. Since suffix
probing is nowhere reliable and only one format depends on in which has a
comment that qemu doesn't do the probing either drop the whole
infrastructure.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>