Temporarily remove the ability to parallelize the collapse operation

The ability to parallelize the `cf.Field.collapse` operation with MPI (https://ncas-cms.github.io/cf-python/performance.html#parallelization) was only ever experimental, and needs to be removed so that v3.9.0 can run on all platforms.

This functionality will return in a robust fashion soon, suitable for operation use, when the move to dask (away from LAMA) has been completed (#182).