How safe is to use BACKUP LOG WITH TRUNCATE_ONLY
What actually doing this code in the SQL-Job? Does it really help?
  


SQL is backing up the log to nowhere and then trying to shrink the data and log files of your database. All of these are bad ideas. Backing up the log to nowhere means you lose point-in-time recovery of your database because you cannot restore these portions of the log. See this link for more info.


  
  Is there anything related performance of the DB-Server?
  


Constantly shrinking your log and data files can cause performance issues because SQL Server has to regrow these files as needed, which causes waits on your queries that require the additional space.


  
  If I remove first line of code (BACKUP LOG), how it can get effect?
  


Before you can reclaim space in the log, you need to backup the data in the log. Removing this would mean your shrinking wouldn't likely free much space, however, as stated before shrinking the log is a bad idea anyway.


  
  Currently I am not able see SQL-Job history of same SQL-Job, is this the reason that it clears all logs?
  


No, this is unrelated. Most likely your SQL Agent history is limited and you have a high-frequency job filling the job history tables. See this link.