Fixes #22395: Remove unused save() override on ManagedFileForm
The method wrote uploaded files to disk via a raw open(), but no code
path reached it. Its only subclass, ScriptFileForm, overrode save() to
write through django-storages and explicitly skipped the base via
super(ManagedFileForm, self).save(). With the override gone, that call
simplifies back to a plain super().save(). A leftover from #18680, which
moved both upload paths onto django-storages but left the form-level
write in place.