xen-pciback: Remove unused function declarations

Commit a92336a1 ("xen/pciback: Drop two backends, squash and cleanup some code.")
declared but never implemented these functions.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230808150912.43416-1-yuehaibing@huawei.com


Signed-off-by: Juergen Gross <jgross@suse.com>