There's a dummy script file you could load: Default.bas - it's the "default" script which is selected on a fresh CHDK installation, located in the CHDK\scripts folder...
But - there's no need to "unload" a script; in the CHDK config file the last used/loaded script ist stored, just for convenience, to run a script again without loading it again.
In <Alt> the one & only function of the shutter button is to start a script...
There's only one case where "unloading" (or a dummy script) could be reasonable: to prevent the accidentally start of a script.
For this you can use a script with just one single line: