[NFC] Remove stale comment about unsafe casts (#7665)

We no longer support unsafe casts, so this comment is not meaningful.k

Author: tlively

src/passes/OptimizeCasts.cpp

@@ -342,11 +342,6 @@ struct EarlyCastFinder
           // change the best cast to move.
           bestMove.bestCast = curr;
         }
-        // We don't care about the safety of the cast at present. If there are
-        // two casts with the same type one being safe and one being unsafe, the
-        // first cast that we visit will be chosen to be moved. Perhaps in the
-        // future we can consider prioritizing unsafe casts over safe ones since
-        // users may be more interested in that.
       }
     }
   }