examples: drop warning silencers no longer hit

Also:
- scope clang `-Wcast-function-type-strict` silencing, add missed `pop`.

Follow-up to d06b49d8b2 #18260

Closes #20896

docs/examples/usercertinmem.c

@@ -40,10 +40,6 @@
 
 #include <curl/curl.h>
 
-#if defined(__GNUC__) || defined(__clang__)
-#pragma GCC diagnostic ignored "-Woverlength-strings"
-#endif
-
 static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *stream)
 {
   fwrite(ptr, size, nmemb, (FILE *)stream);