remove exp/ClimaCore/atm-ocn-lnd

[juliasloan25]

We have an out-dated experiment in experiments/ClimaCore/atm-ocn-lnd/ which couples an old version of Oceananigans with a dummy atmosphere (and land). Since this example hasn't been maintained, it no longer works and is quite out of date. We've decided that since this test case mostly tests ClimaCore functionality, and doesn't use the current source code of the coupler, we should remove it.

Soon (Q2/24?) we'll implement a simple column model (SCM) coupling ClimaAtmos and Oceananigans, which will be a more valuable test case using our actual ClimaCoupler infrastructure.

see branch attempting to update this example: https://github.com/CliMA/ClimaCoupler.jl/tree/ln/revamp-atm-ocn-lnd

closed by #548