Remove dot-keys usage warning and abort compact keys plan.

[omry]

OmegaConf 2.0 started to warn about usage of keys with dot (a.b.c: 10) in preparation for compact keys (#152).
Turns out this is causing too many issues and the pressure for compact keys went down significantly:

1. Hydra's packages serves almost the exact same purpose with a less disruptive change to the yaml semantics.
2. Hydra itself is already using dot keys for package override, for example:

defaults:
 - [email protected]: bar

Would place the content of foo/bar.yaml in the package a.b.c ({a:{b:{c: ... }}}).
This is a dot key that is currently issuing a warning from OmegaConf.

For these reasons, I decided to abandon the idea of compact key support in OmegaConf and I am removing warning on usage of dot keys.