utils: removing diff key from result dictionary

dvc/utils/diff.py

@@ -26,8 +26,7 @@ def _diff_vals(old, new, with_unchanged):
     res = {"old": old, "new": new}
     if isinstance(new, (int, float)) and isinstance(old, (int, float)):
         res["diff"] = new - old
-    elif isinstance(new, str) and isinstance(old, str):
-        res["diff"] = ""
+    
     return res