perf: Drop stateBuffer from Core CKF (#3458)

I noticed that we recreate this buffer for every `findTracks` call which seems to be a massive overhead at the moment. Not sure if we want to pass it in from outside or just use the given container to store these temporary states in. The user will copy out the track and clear the container anyways which keeps memory allocated and should leave enough space for consecutive calls.

Core/include/Acts/TrackFinding/CombinatorialKalmanFilter.hpp

@@ -255,9 +255,6 @@ struct CombinatorialKalmanFilterResult {
   /// Indices into `tracks` which mark active branches
   std::vector<TrackProxy> collectedTracks;
 
-  /// This is used internally to store candidate trackstates
-  std::shared_ptr<TrackStateContainerBackend> stateBuffer;
-
   /// Track state candidates buffer
   std::vector<TrackStateProxy> trackStateCandidates;
 
@@ -364,8 +361,6 @@ class CombinatorialKalmanFilter {
         trackStateCandidates.reserve(std::distance(slBegin, slEnd));
       }
 
-      bufferTrajectory.clear();
-
       // Calibrate all the source links on the surface since the selection has
       // to be done based on calibrated measurement
       for (auto it = slBegin; it != slEnd; ++it) {
@@ -734,7 +729,7 @@ class CombinatorialKalmanFilter {
             Acts::Result<CkfTypes::BranchVector<TrackIndexType>>;
         TrackStatesResult tsRes = trackStateCandidateCreator(
             state.geoContext, *calibrationContextPtr, *surface, boundState,
-            slBegin, slEnd, prevTip, *result.stateBuffer,
+            slBegin, slEnd, prevTip, *result.trackStates,
             result.trackStateCandidates, *result.trackStates, logger());
         if (!tsRes.ok()) {
           ACTS_ERROR(
@@ -1290,7 +1285,6 @@ class CombinatorialKalmanFilter {
             .template get<CombinatorialKalmanFilterResult<track_container_t>>();
     r.tracks = &trackContainer;
     r.trackStates = &trackContainer.trackStateContainer();
-    r.stateBuffer = std::make_shared<TrackStateContainerBackend>();
 
     auto rootBranch = trackContainer.makeTrack();
     r.activeBranches.push_back(rootBranch);