KNOWN_BUGS: fixed "5.6 Improper use of Autoconf cache variables"

As of commit d9f3b365a3

docs/KNOWN_BUGS

@@ -43,7 +43,6 @@ problems may have been fixed or changed somewhat since this was written!
  5.3 libidn and old iconv
  5.4 AIX shared build with c-ares fails
  5.5 can't handle Unicode arguments in Windows
- 5.6 Improper use of Autoconf cache variables
 
  6. Authentication
  6.1 NTLM authentication and unicode
@@ -265,13 +264,6 @@ problems may have been fixed or changed somewhat since this was written!
   https://curl.haxx.se/bug/?i=345
   https://curl.haxx.se/bug/?i=731
 
-5.6 Improper use of Autoconf cache variables
-
- A number of macros used in curl's configure modify the Autoconf cache
- variables which breaks configure variable caching.
-
- Issue: https://github.com/curl/curl/issues/603
-
 
 6. Authentication