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/netrc.c

@@ -26,10 +26,14 @@
 #ifndef CURL_DISABLE_NETRC
 
 #ifdef HAVE_PWD_H
+#ifdef __AMIGA__
 #undef __NO_NET_API /* required for AmigaOS to declare getpwuid() */
+#endif
 #include <pwd.h>
+#ifdef __AMIGA__
 #define __NO_NET_API
 #endif
+#endif
 
 #include <curl/curl.h>
 #include "netrc.h"