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/bufref.c

@@ -30,7 +30,9 @@
 #include "curl_memory.h"
 #include "memdebug.h"
 
+#ifdef DEBUGBUILD
 #define SIGNATURE 0x5c48e9b2    /* Random pattern. */
+#endif
 
 /*
  * Init a bufref struct.