krb5: drop support for Kerberos FTP

It was accidentally broken in commit 0f4c439fc7, shipped since
8.8.0 (May 2024) and yet not a single person has noticed or reported,
indicating that we might as well drop support for FTP Kerberos.

Krb5 support was added in 54967d2a3a (July 2007), and we have
been carrying the extra license information around since then for this
code. This commit removes the last traces of that code and thus we can
remove the extra copyright notices along with it.

Reported-by: Joshua Rogers
Closes #18577

lib/Makefile.inc

@@ -209,7 +209,6 @@ LIB_CFILES =         \
   idn.c              \
   if2ip.c            \
   imap.c             \
-  krb5.c             \
   ldap.c             \
   llist.c            \
   macos.c            \
@@ -292,7 +291,6 @@ LIB_HFILES =         \
   curl_gethostname.h \
   curl_gssapi.h      \
   curl_hmac.h        \
-  curl_krb5.h        \
   curl_ldap.h        \
   curl_md4.h         \
   curl_md5.h         \