Revert all special conversion logic in OmegaConfigLoader for DictConfig

[noklam]

Description

Short description of the problem here.

This is nearly there but I think there's a bug with it unless I'm missing something: what happens if you have a single parameters.yml file and want to use runtime_params?

As part of a separate PR I think we should revert #2378 (but keep test_no_DictConfig_in_store as it's a good regression test) and #2176.

As part of the conversation in #2467, we should revert the changes that we added in #2378 and #2176 as we no longer need to deal with the special DictConfig object. Historically we need that because OmegaConfigLoader return DictConfig instead of a primitive Dict.

We also need to keep the test to make sure there is no regression and #2357 will not happen again.

Context

How has this bug affected you? What were you trying to accomplish?