backup: drop broken BACKUP_FORMAT_DIR

backup: drop broken BACKUP_FORMAT_DIR

Since upstream QEMU 8.0, it's no longer possible to call
bdrv_img_create() from a coroutine anymore, meaning a backup with the
directory format would crash the QEMU instance.

The feature is only exposed via the monitor and was intended to be
experimental. There were no user reports about the breakage and it
only was noticed during the rebase for QEMU 8.1, because other parts
of the backup code needed adaptation and I decided to check the
BACKUP_FORMAT_DIR case too.

It should not stay in a broken state of course, but avoid the
maintenance cost and just make it a removed feature for Proxmox VE 8
retroactively.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>