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/progress.c

@@ -30,9 +30,6 @@
 #include "transfer.h"
 #include "curlx/strcopy.h"
 
-/* check rate limits within this many recent milliseconds, at minimum. */
-#define MIN_RATE_LIMIT_PERIOD 3000
-
 #ifndef CURL_DISABLE_PROGRESS_METER
 /* Provide a string that is 7 letters long (plus the zero byte).