elixir/cowboy: remove mysql config to fix warnings
The app prints the following warnings on Elixir 1.2:
> You have configured application :mysql in your configuration
> file but the application is not available.
>
> This usually means one of:
>
> 1. You have not added the application as a dependency in a mix.exs file.
>
> 2. You are configuring an application that does not really exist.
>
> Please ensure :mysql exists or remove the configuration.
This app isn't using the config at all, so instead of changing the key,
I'm removing it.
Utkarsh Kukreti