Blocks page: Remove CSS that hides 2nd column on mobile.

[jasmussen]

Related to #438 (comment)

On the mobile breakpoint, the illustration disappears:

With the zero bottom margin, it looks a bit tight. There appears to be some custom CSS that hides this:

@media (max-width: 781px) {
    #intro .wp-block-column:last-child {
        display: none;
    }
}

If we can remove that, it would look like this, which works pretty well:

[ryelle]

That CSS is used on the homepage to prevent the brushstroke from wrapping. Instead, let's remove the intro ID on this section.