pop3: Removed bytecountp from the per-request structure

Removed this pointer to a downloaded bytes counter because it was set in
pop3_init() to point to the same variable the transfer functions keep
the count in (k->bytecount), effectively making the code in transfer.c
"*k->bytecountp = k->bytecount" a no-op.

lib/pop3.h

@@ -57,7 +57,6 @@ typedef enum {
    perhaps the SessionHandle is changed between the times the connection is
    used. */
 struct POP3 {
-  curl_off_t *bytecountp;
   curl_pp_transfer transfer;
   char *id;               /* Message ID */
   char *custom;           /* Custom Request */