refactor: remove unused model parameters

board.virtioblockdevice.image_path parameter is never set in affected
models. It assumed to be empty by default by respective models, removing
not to clutter configuration files. Affected models:
css-rdn1edge.sh
css-rdn1edgex2.sh
css-rdv1.sh
css-sgi575.sh
morello.sh

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: I8b66d4d1c7d000dbcf9e6bf06b71391650aed8c9
5 files changed