ksmbd: remove ksmbd_vfs_copy_file_range

vfs_copy_file_range and cifs client already does this type of fallback,
so this is dead code.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>