clang-tidy: drop redundant casts

Found via `readability-redundant-casting`. Prone to false positives, not
enabled.

Ref: https://clang.llvm.org/extra/clang-tidy/checks/readability/redundant-casting.html

Closes #20630

docs/examples/chkspeed.c

@@ -63,7 +63,7 @@ static size_t write_cb(void *ptr, size_t size, size_t nmemb, void *data)
      so we only return the size we would have saved ... */
   (void)ptr;
   (void)data;
-  return (size_t)(size * nmemb);
+  return size * nmemb;
 }
 
 int main(int argc, const char *argv[])

docs/examples/ftpgetinfo.c

@@ -42,7 +42,7 @@ static size_t throw_away(void *ptr, size_t size, size_t nmemb, void *data)
   (void)data;
   /* we are not interested in the headers itself,
      so we only return the size we would have saved ... */
-  return (size_t)(size * nmemb);
+  return size * nmemb;
 }
 
 int main(void)