Cleanup old banner templates

[dencold]

In 61da5f477410dfa3a0a64736b4984fc35d3bc962 we consolidated & simplified our "banner" templates into the new cover template logic. However we still have the old files hanging around:

• banner.html
• banner_full.html

I believe that there are still some templates that depend on the banner.html partial.

• [x] check through theme for any dependencies
• [x] remove and switch to cover partials where needed
• [x] remove the two files above