Remove old Objects in scene

Okay. I'm making an infinit runner. But I can't figure out how to remove objects that I've run past. I am very sorry if my code is bad. This is my first game. I need a no-lag solution. :)

IF you want to see what the game looks like:

HERE IS THE CODE:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class generation : MonoBehaviour {
 
     
     private float speed = 5f;
     public static float groundNumber = 10;
     private float passedNumber = 0f;
     GameObject startPlatform;
     GameObject gro

     public float id = 0f;
 
 
     //PREFABS FOR INSTANSIATE
     public static GameObject wallPrefab;
     public static float genNumber;
     public static GameObject groundPrefab;
     
    
     // Use this for initialization
     
     
     void Start () {
 
         startPlatform = GameObject.Find("StartingPlatform");
         wallPrefab = GameObject.Find("Wall");
         groundPrefab = GameObject.Find("StartingPlatform");
         gameObject.GetComponent<playerController>();
         wallPrefab.GetComponent<Renderer>().material.color = Color.green;
         groundPrefab.GetComponent<Renderer>().material.color = Color.white;
         groundNumber = 10;
         neverRepeat = false;
         neverRepeat2 = false;
         neverRepeat3 = false;
         neverRepeat4 = false;
 
     }
 
 
     public static bool neverRepeat = false;
     public static bool neverRepeat2 = false;
     public static bool neverRepeat3 = false;
     public static bool neverRepeat4 = false;
     private int wall = 0;
     // Update is called once per frame
     void DestroyPlatform ()
     {
 
     }
     void FixedUpdate () {
 
         #region Ground
 
         GameObject ground = GameObject.CreatePrimitive(PrimitiveType.Cube);
         ground.transform.position = new Vector3(0, -1, 0);
         ground.transform.localScale = new Vector3(10, 1, 10);
             ground.tag = "platformNumber";
         
         
         ground.transform.position = new Vector3(0, -1, groundNumber);
         ground.GetInstanceID();
 
        
         groundNumber += 10;
         
         if (groundNumber > 1000 && neverRepeat == false)
         {  
             Debug.Log("Ground Number is more than 1000");
             neverRepeat = true;
             playerController.movementSpeed += 2f;
             id = +1;
 
         }
 
         if (groundNumber > 3000 && neverRepeat2 == false)
         {
             Debug.Log("Ground Number is more than 3000");
             neverRepeat2 = true;
             playerController.sideMoveSpeed += 0.01f;
             playerController.movementSpeed += 2f;
         }
         if (groundNumber > 5000 && neverRepeat3 == false)
         {
             Debug.Log("Ground Number is more than 5000");
             neverRepeat3 = true;
             playerController.movementSpeed += 1f;
         }
         if (groundNumber > 7000 && neverRepeat4 == false)
         {
             Debug.Log("Ground Number is more than 7000");
             neverRepeat4 = true;
             playerController.sideMoveSpeed += 0.01f;
             playerController.movementSpeed += 1f;
         }
 
         #endregion
         
         
         Vector3 wallPOS = new Vector3(Random.Range(-5f, 5f), 0, Random.Range(-5f, 5f) + id);
         Instantiate(wallPrefab, wallPOS, Quaternion.identity);
         id += 10;
 
 
 
 
     }
 
 }