idn: drop support for libidn

The current tree doesn't even compile with libidn(1) after
2c7bdaf9f1, which included
a non-existent call to check_dlopen_blocked() somehow.
Hence, it feels safe to just nuke legacy support from
our repo.

src/basic/build.c

@@ -120,12 +120,6 @@ const char* const systemd_features =
         " -IDN2"
 #endif
 
-#if HAVE_LIBIDN
-        " +IDN"
-#else
-        " -IDN"
-#endif
-
 #if HAVE_KMOD
         " +KMOD"
 #else