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/content_encoding.c

@@ -65,7 +65,10 @@
 
 /* allow no more than 5 "chained" compression steps */
 #define MAX_ENCODE_STACK 5
+
+#if defined(HAVE_LIBZ) || defined(HAVE_BROTLI) || defined(HAVE_ZSTD)
 #define DECOMPRESS_BUFFER_SIZE 16384 /* buffer size for decompressed data */
+#endif
 
 #ifdef HAVE_LIBZ