Remove obsolete TODO

PiperOrigin-RevId: 640138527
Change-Id: I650dd91d76824cfd3a57b36eca282f41b3667125

absl/strings/internal/cord_internal.h

@@ -399,7 +399,6 @@ inline CordRepSubstring* CordRepSubstring::Create(CordRep* child, size_t pos,
   assert(pos < child->length);
   assert(n <= child->length - pos);
 
-  // TODO(b/217376272): Harden internal logic.
   // Move to strategical places inside the Cord logic and make this an assert.
   if (ABSL_PREDICT_FALSE(!(child->IsExternal() || child->IsFlat()))) {
     LogFatalNodeType(child);