fpu: Remove default handling for dnan_pattern

Now that all our targets have bene converted to explicitly specify
their pattern for the default NaN value we can remove the remaining
fallback code in parts64_default_nan().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241202131347.498124-55-peter.maydell@linaro.org