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/http.c

@@ -164,9 +164,6 @@ char *Curl_checkProxyheaders(struct Curl_easy *data,
 
   return NULL;
 }
-#else
-/* disabled */
-#define Curl_checkProxyheaders(x, y, z, a) NULL
 #endif
 
 static bool http_header_is_empty(const char *header)