FTP: remove krb4 support

We've announced this pending removal for a long time and we've
repeatedly asked if anyone would care or if anyone objects. Nobody has
objected. It has probably not even been working for a good while since
nobody has tested/used this code recently.

The stuff in krb4.h that was generic enough to be used by other sources
is now present in security.h

lib/krb5.c

@@ -51,7 +51,7 @@
 #include "ftp.h"
 #include "curl_gssapi.h"
 #include "sendf.h"
-#include "krb4.h"
+#include "security.h"
 #include "curl_memory.h"
 #include "warnless.h"