Remove legacy_server_opt stats counter

Summary:
Remove the `legacy_server_opt` statistics counter from TPR structs and all related logging/tracking code.

The legacy stats counter was used during the rollout of the new TPR server optimization. Since the rollout is complete and only `new_server_opt` is needed for monitoring, the legacy counter creates unnecessary overhead and code complexity. This is the first step in a cleanup - the next commit will rename `new_server_opt` to `server_opt` for clarity purposes.

Simplifies TPR codebase by removing deprecated metrics. Reduces memory footprint of TPR stats structures and eliminates confusion between legacy vs new server optimization metrics.

Reviewed By: nikhildl12

Differential Revision: D80843067

fbshipit-source-id: 9746f24cde34b278e7da1071fb86ec26c8d7ba33

Author: p-shah256

katran/tpr/TPRStatsPoller.cpp

@@ -41,7 +41,6 @@ void TPRStatsPoller::setStatsCounters(const tcp_router_stats& stats) {
   setCounter("error_sys_calls", stats.error_sys_calls);
   setCounter("ignoring_due_to_kde", stats.ignoring_due_to_kde);
   setCounter("new_server_opt", stats.new_server_opt);
-  setCounter("legacy_server_opt", stats.legacy_server_opt);
 }
 
 TPRStatsPoller::TPRStatsPoller(
@@ -159,7 +158,6 @@ TPRStatsPoller::collectTPRStats(int numCpus) {
     aggregateStats.error_write_opt += stat.error_write_opt;
     aggregateStats.error_sys_calls += stat.error_sys_calls;
     aggregateStats.ignoring_due_to_kde += stat.ignoring_due_to_kde;
-    aggregateStats.legacy_server_opt += stat.legacy_server_opt;
     aggregateStats.new_server_opt += stat.new_server_opt;
   }
   return aggregateStats;

katran/tpr/TPRTypes.h

@@ -46,7 +46,6 @@ struct tcp_router_stats {
   uint64_t error_write_opt{0};
   uint64_t error_sys_calls{0};
   uint64_t ignoring_due_to_kde{0};
-  uint64_t legacy_server_opt{0};
   uint64_t new_server_opt{0};
 };
 
@@ -96,7 +95,6 @@ inline std::ostream& operator<<(std::ostream& os, const tcp_router_stats& s) {
      << " error_server_id_zero=" << s.error_server_id_zero
      << " error_write_opt=" << s.error_write_opt
      << " error_sys_calls=" << s.error_sys_calls
-     << " legacy_server_opt=" << s.legacy_server_opt
      << " new_server_opt=" << s.new_server_opt;
   return os;
 }

katran/tpr/bpf/tcp_pkt_router_structs.h

@@ -42,6 +42,5 @@ struct stats {
   __u64 error_sys_calls;
   __u64 ignoring_due_to_kde;
   // stats for the rollout of the new TPR server OPT
-  __u64 legacy_server_opt;
   __u64 new_server_opt;
 };