libcacard/cac: Remove unused statement (value stored is never read)

Warning from ccc-analyzer:

libcacard/cac.c:192:13: warning: Value stored to 'ret' is never read
            ret = VCARD_DONE;
            ^     ~~~~~~~~~~

Here 'ret' is assigned a value inside of a switch statement and also after
that switch statement.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

libcacard/cac.c

@@ -189,7 +189,6 @@ cac_applet_pki_process_apdu(VCard *card, VCardAPDU *apdu,
             pki_applet->sign_buffer = sign_buffer;
             pki_applet->sign_buffer_len = size;
             *response = vcard_make_response(VCARD7816_STATUS_SUCCESS);
-            ret = VCARD_DONE;
             break;
         case 0x00:
             /* we now have the whole buffer, do the operation, result will be