KNOWN_BUGS: remove libidn related issue

... as we no longer use libidn
diff --git a/docs/KNOWN_BUGS b/docs/KNOWN_BUGS
index b438010..2405ee9 100644
--- a/docs/KNOWN_BUGS
+++ b/docs/KNOWN_BUGS
@@ -43,7 +43,6 @@
  5. Build and portability issues
  5.1 Windows Borland compiler
  5.2 curl-config --libs contains private details
- 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 cmake support gaps
@@ -303,14 +302,6 @@
  run that might be needed only for building libcurl. Further, curl-config
  --cflags suffers from the same effects with CFLAGS/CPPFLAGS.
 
-5.3 libidn and old iconv
-
- Test case 165 might fail on a system which has libidn present, but with an
- old iconv version (2.1.3 is a known bad version), since it doesn't recognize
- the charset when named ISO8859-1. Changing the name to ISO-8859-1 makes the
- test pass, but instead makes it fail on Solaris hosts that use its native
- iconv.
-
 5.4 AIX shared build with c-ares fails
 
  curl version 7.12.2 fails on AIX if compiled with --enable-ares.  The