Remove block-body exception from coding convention

#115

We have been working on using ESLint to help highlight coding convention infractions instead of folks either having to remember them when submitting or reviewing PRs. In [Ensure use of block body for arrow functions](DEFRA/water-abstraction-system#1006) we managed to figure out how to have the tool highlight when someone has used concise over block-body format for an arrow function.

However, the rule cannot distinguish between when it was being used normally, or just to check if something is 'truthy'. That was the one exception we had and there is no point in keeping it documented here if the tool won't allow. We also didn't think it was worth the effort to try and 'hack' ESLint or develop a custom rule just to support this. On balance, its goes against one of our guiding principle of consistency anyway.

So, this change removes the exceptions from our docs.