ui: remove check for failure of qemu_acl_init()

The qemu_acl_init() function has long since stopped being able
to return NULL, since g_malloc will abort on OOM. As such the
checks for NULL were unreachable code.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
