don't mention it vs forget it

don't mention it

phrase
  • A polite way of responding to thanks, by suggesting that the service is too trivial or humble to warrant thanks; you're welcome. 

  • Used other than figuratively or idiomatically: see don't, mention, it. 

forget it

verb
  • Don't let it bother you; put it out of your mind. 

  • I don't want to continue with this conversation (usually used in frustration or anger). 

  • It doesn't matter. 

  • A conventional reply to an expression of gratitude. 

  • Absolutely not; I forbid it or it is impossible. 

How often have the words don't mention it and forget it occurred in a corpus of books? (source: Google Ngram Viewer )