tidy-up: delete, comment or scope C macros reported unused

To reduce the number `-Wunused-macro` compiler warnings:
- delete unused macros.
- comment out unused macro that are part of a set.
- move macros into the scope they are used.

This may be useful to enable by default, but there are tricky cases that
I didn't manage to fix and paused the effort. E.g. internal features
checks in `openssl.c`. There is more, once those are fixed.

Closes #16279

lib/version.c

@@ -422,8 +422,6 @@ static int idn_present(curl_version_info_data *info)
   return info->libidn != NULL;
 #endif
 }
-#else
-#define idn_present     NULL
 #endif
 
 #if defined(USE_SSL) && !defined(CURL_DISABLE_PROXY) && \