remove duplicate main element and IDs

[iamjolly]

the default layout calls in the page layout inside a <main> element, so the main inside the page layout needed to be removed.

[iamjolly]

Now I'm wondering if I got the problem backwards...

• page layouts have a main element inside the content, while post layouts do not have a main.
• The default layout uses a conditional which puts everything except a post inside its own main content.

I think the default layout conditional - linked here should instead be putting anything except a page inside the main.

I could use another pair of brain hemispheres from someone like @thisisdano or @afeld to tell me if this makes sense. If so, then my PR can be closed without merging, and I'll open a new one with the one change to the default layout.