Welcome to TSG!
Can't do it in Notepad, but
Notepad++ can do it easily. A great free Notepad replacement.
Highlight the line including the CR/LF by clicking in the border to the left of the line.
If it's not the entire line, just highlight to the end and bring the mouse back to the start of the next line, but don't highlight the first letter of the next line.
Press
CTRL+H for Replace, leave the
Replace With box blank, click
Replace All.