HTML export: exclude computation messages from the output.

[alshan]

For example, if "sampling" computation messages are present:

ggplot() + geom_point(aes(
    x = [v for v in range(10)],
    y = [v for v in range(10)],
), sampling=sampling_random(5))

then generated by ggsave() HTML includes the "sampling" message but trims the plot bottom:

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At the same time SVG, PNG export works correctly, i.e. computation messages aren't included to the output.