remove dead split_labels helper

Its only caller was the deleted yoloworld_onnx.cc.

src/engines/yolo_helpers.h

@@ -5,8 +5,6 @@
 
 #include <algorithm>
 #include <cmath>
-#include <sstream>
-#include <string>
 #include <vector>
 
 namespace engines::detail {
@@ -135,19 +133,4 @@ inline void backproject(float u, float v, float z,
     Y = (v - cy) * z / fy;
 }
 
-// --- Label parsing ---
-
-inline std::vector<std::string> split_labels(const std::string& s) {
-    std::vector<std::string> out;
-    std::stringstream ss(s);
-    std::string tok;
-    while (std::getline(ss, tok, ',')) {
-        size_t l = tok.find_first_not_of(" \t");
-        size_t r = tok.find_last_not_of(" \t");
-        if (l != std::string::npos)
-            out.push_back(tok.substr(l, r - l + 1));
-    }
-    return out;
-}
-
 } // namespace engines::detail