The general principle whichever editor you're using is that you need to include the new line characters (
\r\n assuming Windows line endings) in your search so that they will be included in the replacement and thus removed.
In Notepad++ this is made a bit more confusing by the 2 different search and replace commands. See this Wiki page for details: Replacing Newlines in Notepad++
I only had an old version of Notepad++ to hand so had to use Extended Search (accessed via ^R), making sure Regular Expr was unticked and using ^M to insert the new line character into the Find field. However if you make sure you're using Notepad++ 4.9 or later you should be able to use
\n in the regexp mode.