[googlefonts/vertical_metrics] Some checks are redundant

[simoncozens]

We check:

    # OS/2.usWinAscent must be strictly positive
    if font_metrics["OS/2.usWinAscent"] <= 0:
        yield FAIL, Message(
            "win-ascent",
            f"The OS/2.usWinAscent must be strictly positive,"
            f" but the font has {font_metrics['OS/2.usWinAscent']}",
        )

    # OS/2.usWinDescent must be positive or zero
    if font_metrics["OS/2.usWinDescent"] < 0:
        yield FAIL, Message(
            "win-descent",
            "The OS/2.usWinDescent must be positive or zero."
            " This font has a negative value.",
        )

These values are stored in the font as unsigned values; they can never be negative.