build: fix -Wunused-macros warnings, and related tidy-ups

- fix internal macro `AN_APPLE_OS` reused between sources without
  resetting it. It may potentially have left the system sha256
  function unused.
- fix to define `WOLFSSL_OPTIONS_IGNORE_SYS` so that it always applies
  to wolfSSL headers, also during feature detection.
- md4, md5, sha256: simplify fallback logic.
- delete 20+ unused macros.
- scope or move macros to avoid `-Wunused-macros` warnings.
- examples: delete unused code.

The warning detects macros defined but not used within the same C
source. It does not warn for macros defined in headers. It also works
with unity builds, but to a lesser extent.

Closes #20593

lib/setopt.c

@@ -188,8 +188,10 @@ static CURLcode setstropt_interface(char *option, char **devp,
   return CURLE_OK;
 }
 
+#ifdef USE_SSL
 #define C_SSLVERSION_VALUE(x)     (x & 0xffff)
 #define C_SSLVERSION_MAX_VALUE(x) ((unsigned long)x & 0xffff0000)
+#endif
 
 static CURLcode protocol2num(const char *str, curl_prot_t *val)
 {