Fix a leftover optional argument in the tsickle/ngc integration.
Fix a leftover optional argument in the tsickle/ngc integration.

getGeneratedExterns was changed to take an optional rootDir a while ago, with
the intent of later changing ngc to pass the argument. This change makes the
argument non-optional and removes the special case.

Prerequisite is https://github.com/angular/angular/pull/44768 landing on the
ngc side.

PiperOrigin-RevId: 423257024

angular/tsickle,angular/tsickle