As Acry mentioned, you can just restore the original files.
Level boost alters the levels permanently, it's not just turning on a flag that can be turned off.
Ie, level 20 -> level 20 + 50% -> level 30
level 30 is now saved instead of level 20
If you wanted to undo this, simply set the level boost to -66 percent (which is ~100/150) to scale them back down to pretty much the original level.
The level boost can be used as a level decreaser too