Issue #3617944: Remove unnecessary string cast in DateTimePlus::__toString()

DateTimePlus::render() is documented (@return string) and always returns a string, so casting its result to string in __toString() has no effect.

Scoped to DateTimePlus only. The existing MR (!16827) also touched TranslatableMarkup::__toString(), which broke LinkBaseTest because Renderer output there can genuinely be null, not just documented as string; that part needs separate investigation and is left out of this change.

Closes #3617944