CI 3.1.5 php Warning shell_exec

Hello,

I got this warning:

A PHP Error was encountered

Severity: Warning

Message: shell_exec() has been disabled for security reasons

Filename: models/Magazine_model.php

Line Number: 82

Backtrace:

File: /home/****/public_html/application/models/Magazine_model.php
Line: 82
Function: shell_exec

File: /home/****/public_html/application/controllers/Magazine.php
Line: 29
Function: get_highest_views_posts

File: /home/***/public_html/index.php
Line: 315
Function: require_once

First I thought files are infected with bad virus codes, I found shell_exec exists in the file
system/libraries/upload.php

I didn't use any shell_exec command or upload operations. this is the model function centent:

PHP Code:

function get_highest_views_posts() {
    $this->db->select('fid,tid,subject,dateline,replies,views,visible,username,closed');
    $this->db->order_by(`views`, 'asc');
    $this->db->where_in('closed', Array('0','1',''));
    $this->db->where_not_in('fid', $this->config->item('private_forums'));
    $this->db->limit($this->config->item('posts_views_limit'));
    $query = $this->db->get('threads');
    return $query->result();
  } 


Is there another solution to solve this than setting environment to production to hide warnings?

Regards,