Compatibility with optparse-applicative HEAD

[Bodigrim]

https://github.com/pcapriotti/optparse-applicative/blob/db1bf6556afbe8f2a238b811fcd303f4541900c7/CHANGELOG.md says:

## Version 0.20.0.0 (Unreleased)
...
- Remove `fullDesc` and `briefDesc` builder modifiers – they have not had an
  effect since version 0.8.

We don't have to wait until optparse-applicative-0.20 gets released, we can scrap no-op fullDesc already.