[keycloak-dev] Removing providers directory

[Stian Thorgersen]

I'm planning on removing the providers directory. Any objections?

At the moment we have 3 approaches to deploy custom providers:

* 'deployments' dir
* 'providers' dir
* module

The deployments and providers are similar, but deployments is much nicer
and more powerful. So we should just remove the providers directory.

Modules needs to stay at least for now as that is the only way to deploy
custom SPIs.