remove outdated src/ folders

[juliasloan25]

We have two folders src/CoupledSimulations/ and src/CouplerState, which contain placeholder code that isn't needed anymore. We should remove these folders, the files in them, and remove/modify files that refer to them as necessary.

More details here: #455 (comment)

Update: Replacing this usage in the seabreeze experiment will be more involved than we expected, and isn't a huge priority right now. Instead of completely removing these two folders, we'll move the code in them to experiments/ClimaCore/ to emphasize that they aren't part of the main codebase, but are still used for these experiments. Eventually we want these experiments to test our maintained functionality, but this is a temporary solution for now.

#468 moves these files into experiments/ClimaCore/