Remove TODO comments which are no longer needed (sonic-net#2568)

What I did
Remove TODO comments which are no longer needed

Why I did it
MUX support has already been removed, the TODO item is not valid

How I verified it
Only comment change

orchagent/flex_counter/flowcounterrouteorch.h

@@ -141,7 +141,7 @@ class FlowCounterRouteOrch : public Orch
     RouterFlowCounterCache mBoundRouteCounters;
     /* Cache for those route flow counters pending update to FLEX DB */
     RouterFlowCounterCache mPendingAddToFlexCntr;
-    /* IP2ME, MUX */ // TODO: remove MUX support
+    /* IP2ME */
     MiscRouteEntryMap mMiscRoutes; // Save here for route flow counter
     /* Flex counter manager for route flow counter */
     FlexCounterManager mRouteFlowCounterMgr;