smb: client: Remove an unused function and variable

SMB2_QFS_info() has been unused since 2018's
commit 730928c8 ("cifs: update smb2_queryfs() to use compounding")

sign_CIFS_PDUs has been unused since 2009's
commit 2edd6c5b ("[CIFS] NTLMSSP support moving into new file, old dead
code removed")

Remove them.

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