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

docs/cmdline-opts/krb.md

@@ -6,7 +6,7 @@ Arg: <level>
 Help: Enable Kerberos with security <level>
 Protocols: FTP
 Requires: Kerberos
-Category: ftp
+Category: deprecated
 Added: 7.3
 Multi: single
 See-also:
@@ -18,6 +18,8 @@ Example:
 
 # `--krb`
 
+Deprecated option (added in 8.17.0). It has no function anymore.
+
 Enable Kerberos authentication and use. The level must be entered and should
-be one of 'clear', 'safe', 'confidential', or 'private'. Should you use a
-level that is not one of these, 'private' is used.
+be one of `clear`, `safe`, `confidential`, or `private`. Should you use a
+level that is not one of these, `private` is used.

docs/libcurl/curl_easy_setopt.md

@@ -540,7 +540,7 @@ Client key password. See CURLOPT_KEYPASSWD(3)
 
 ## CURLOPT_KRBLEVEL
 
-Kerberos security level. See CURLOPT_KRBLEVEL(3)
+**OBSOLETE**. Kerberos security level. See CURLOPT_KRBLEVEL(3)
 
 ## CURLOPT_LOCALPORT
 

docs/libcurl/opts/CURLOPT_KRBLEVEL.md

@@ -5,7 +5,6 @@ Title: CURLOPT_KRBLEVEL
 Section: 3
 Source: libcurl
 See-also:
-  - CURLOPT_KRBLEVEL (3)
   - CURLOPT_USE_SSL (3)
 Protocol:
   - FTP
@@ -26,11 +25,13 @@ CURLcode curl_easy_setopt(CURL *handle, CURLOPT_KRBLEVEL, char *level);
 
 # DESCRIPTION
 
+Deprecated. It serves no purpose anymore.
+
 Pass a char pointer as parameter. Set the kerberos security level for FTP;
 this also enables kerberos awareness. This is a string that should match one
-of the following: &'clear', &'safe', &'confidential' or &'private'. If the
-string is set but does not match one of these, 'private' is used. Set the
-string to NULL to disable kerberos support for FTP.
+of the following: `clear`, `safe`, `confidential` or `private`. If the string
+is set but does not match one of these, `private` is used. Set the string to
+NULL to disable kerberos support for FTP.
 
 The application does not have to keep the string around after setting this
 option.
@@ -62,8 +63,14 @@ int main(void)
 
 # HISTORY
 
+Functionality removed in 8.17.0
+
 This option was known as CURLOPT_KRB4LEVEL up to 7.16.3
 
+# DEPRECATED
+
+Deprecated since 8.17.0
+
 # %AVAILABILITY%
 
 # RETURN VALUE

docs/libcurl/symbols-in-versions

@@ -704,7 +704,7 @@ CURLOPT_ISSUERCERT_BLOB         7.71.0
 CURLOPT_KEEP_SENDING_ON_ERROR   7.51.0
 CURLOPT_KEYPASSWD               7.17.0
 CURLOPT_KRB4LEVEL               7.3           7.17.0
-CURLOPT_KRBLEVEL                7.16.4
+CURLOPT_KRBLEVEL                7.16.4        8.17.0
 CURLOPT_LOCALPORT               7.15.2
 CURLOPT_LOCALPORTRANGE          7.15.2
 CURLOPT_LOGIN_OPTIONS           7.34.0