Don't show title twice when showing manual contents

([Trello ticket](https://trello.com/c/lPu0qn8v/259-bug-title-of-manuals-shouldn-t-be-repeated-on-contents-page-1))

Remove the redundant heading (it's displayed in the main bar at the top) and bump the size of the summary text to 24.

Before:
![before](https://dl.dropboxusercontent.com/u/39836361/Screenshot%202014-08-06%2014.18.48.png)

After:
![after](https://dl.dropboxusercontent.com/u/39836361/Screenshot%202014-08-06%2014.14.15.png)
Without explicitly adding the paragraph tag the rule doesn't have enough specificity to override the `.govspeak p` definition [here](https://github.com/alphagov/manuals-frontend/blob/master/app/assets/stylesheets/_govspeak.scss#L40)