Turbopack: remove EcmascriptParsable::parse_original (#94465)
`EcmascriptParsable::parse_original` is used to access the original
module when using 'split modules', the only place that needed this was
already checking for split modules for other reasons so we can simply
remove this function from the trait and access the split module
'original' directly.
Also remove EcmascriptParseable::ty which was dead
