I can't remove an manage_file field image
You can use db query to remove the entry 

  $mydir =  variable_get('file_public_path', conf_path() . '/files');
  $filename = $mydir .$file_directory. $filename;
  unlink($filename);
  db_delete('file_managed')
      -&gt;condition('filename', $filename)
      -&gt;execute();
  $FileStatus = "The file has been removed - ".$filename;


Get the public directory path and delete the file from file_managed table 

This will make your work easy.