This is also possible with Notepad++

• Go to the search menu Ctrl+F and open the "Mark" tab.
• Check "Bookmark line" (if there is no "Mark" tab update to the current version).
• Enter your search term and click "Mark All"
  • All lines containing the search term are bookmarked.
• Now go to the Menu "Search -> Bookmark -> Remove Bookmarked lines"
• Done.