Are there shortcuts for text editors/processors to delete the rest of a line

The feature you're describing is essentially a pretty version of "replace all", which Notepad++ and any decent text editor will have. I'm not aware of Notepad++ having the feature exactly as you describe it, so I'd just do a "Replace All". (btw, I'm using N++ version 5.8.7)

There is no shortcut by default, but you can easily add one using the shortcut mapper. The action to remove leading and trailing space is found in Edit > Blank operations > Remove leading and trailing space

You can map it by going to Settings > Shortcut Mapper... > Then scrolling to whichever one you want (such as 29) and double clicking on the cell under Shortcut.