Repo cleanup: Remove unnecessary directories

[juliasloan25]

We've accumulated a couple of directories that are now unused and no longer need to be maintained, notably experiments/AMIP/moist_mpi_aquaplanet and experiments/AMIP/moist_mpi_earth.

The experiments/AMIP/moist_mpi_earth directory was refactored to become experiments/AMIP/modular and the modules contained in src/, so its functionality is retained there.

Closed by #315

Removed Content Summary

moist_mpi_earth

• AMIP mode
  • atmosphere: ClimaAtmos full model
  • land: ClimaLSM bucket model
  • ocean: uses SST read in from a file
  • ice: uses SIC read in from a file
  • not a closed system, so conservation is not guaranteed
• slabplanet mode
  • atmosphere: ClimaAtmos full model
  • land: ClimaLSM bucket model
  • ocean: thermal slab
  • ice: thermal slab
  • conserves energy

moist_mpi_aquaplanet

• atmosphere: ClimaAtmos full model
• surface: thermal slab, no distinction between land/ocean/ice
• conserves energy

To Do

• Delete these 2 directories
• Add notes in this issue about the purpose of each of these directories for future reference