Remove focus on <main> element

Having a negative tab index allows the user to focus on the main element,
which adds an outline from a user agent stylesheet. This was added in
https://github.com/alphagov/frontend/commit/cd20d6ec2dab5fff2282a60cf1152173030bd5dd
but the 'Skip to content' links behaviour is unchanged in this commit.
Merge pull request #580 from alphagov/remove-focus-on-main-element

Remove focus on <main> element