imap: Removed some FTP heritage leftovers

Removed user and passwd from the IMAP struct as these cannot be set on
a per-request basis and are leftover from legacy FTP code.

lib/imap.h

@@ -58,8 +58,6 @@ typedef enum {
    used. */
 struct IMAP {
   curl_off_t *bytecountp;
-  char *user;             /* User name string */
-  char *passwd;           /* Password string */
   curl_ftptransfer transfer;
   char *mailbox;          /* Mailbox to select */
 };