cifs: Remove pre-historic unused CIFSSMBCopy

CIFSSMBCopy() is unused, remove it.

It seems to have been that way pre-git; looking in a historic
archive, I think it landed around May 2004 in Linus'
BKrev: 40ab7591J_OgkpHW-qhzZukvAUAw9g
and was unused back then.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Steve French <stfrench@microsoft.com>