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/http2.c

@@ -51,10 +51,6 @@
 #error too old nghttp2 version, upgrade!
 #endif
 
-#ifndef CURLVERBOSE
-#define nghttp2_session_callbacks_set_error_callback(x, y)
-#endif
-
 #if (NGHTTP2_VERSION_NUM >= 0x010c00)
 #define NGHTTP2_HAS_SET_LOCAL_WINDOW_SIZE 1
 #endif