tidy-up: delete, comment or scope C macros reported unused

To reduce the number `-Wunused-macro` compiler warnings:
- delete unused macros.
- comment out unused macro that are part of a set.
- move macros into the scope they are used.

This may be useful to enable by default, but there are tricky cases that
I didn't manage to fix and paused the effort. E.g. internal features
checks in `openssl.c`. There is more, once those are fixed.

Closes #16279

lib/vauth/digest.c

@@ -50,6 +50,7 @@
 #include "curl_memory.h"
 #include "memdebug.h"
 
+#ifndef USE_WINDOWS_SSPI
 #define SESSION_ALGO 1 /* for algos with this bit set */
 
 #define ALGO_MD5 0
@@ -59,7 +60,6 @@
 #define ALGO_SHA512_256 4
 #define ALGO_SHA512_256SESS (ALGO_SHA512_256 | SESSION_ALGO)
 
-#if !defined(USE_WINDOWS_SSPI)
 #define DIGEST_QOP_VALUE_AUTH             (1 << 0)
 #define DIGEST_QOP_VALUE_AUTH_INT         (1 << 1)
 #define DIGEST_QOP_VALUE_AUTH_CONF        (1 << 2)
@@ -141,8 +141,8 @@ bool Curl_auth_digest_get_pair(const char *str, char *value, char *content,
   return TRUE;
 }
 
-#if !defined(USE_WINDOWS_SSPI)
-/* Convert md5 chunk to RFC2617 (section 3.1.3) -suitable ASCII string */
+#ifndef USE_WINDOWS_SSPI
+/* Convert MD5 chunk to RFC2617 (section 3.1.3) -suitable ASCII string */
 static void auth_digest_md5_to_ascii(unsigned char *source, /* 16 bytes */
                                      unsigned char *dest) /* 33 bytes */
 {