Revert "Add Defra to the A-Z"

As @BenClancy points out, Defra is in the style guide so doesn't
need to be in the A-Z.

https://github.com/alphagov/design-principles/pull/168#issuecomment-45714658

https://www.gov.uk/design-principles/style-guide/style-points#style-capitals-government-departments

This reverts commit 17bd3e9bec3429b6434ad10d9dde1e912075f5f5.