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

[vszakats]

• 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.

---

• move OPENSSL_SUPPRESS_DEPRECATED to build level, and limit setting
  it for OpenSSL 3 (non-fork). Maybe in a separate PR. (see reverted sub-commit in this PR)
• cmake: limit OPENSSL_SUPPRESS_DEPRECATED to lib and feature checks. [same as above]
• decide to enable the warnings as an error, or just a warning, or an error but only in CI (or not at all.)
  or perhaps enable it in scheduled, non-unity, CI jobs? → SEPERATE PR. I'm thinking maybe enable it as a non-error warning could be useful. If there is good chance to fix remaining fallouts, and avoid future ones via CI, the warning can be made an error.
• consider to avoid MD5_* and EVP_PKEY_get1_RSA, RSA_flags, RSA_free when building with OpenSSL 3,
  to avoid the OPENSSL_SUPPRESS_DEPRECATED warnings in the first place. The logic is already present and used for OpenSSL 3 no-deprecated builds. [RATHER NOT, it'd mean NTLM is no longer available with ossl3]
  This leaves the example [FIXED] and TLS-SRP as the remining reasons. (TLS-SRP is a barely supported, insecure security feature.) [in context of this PR only examples were an issue]