Remove `mpi4py` dependency from conda package build config

In the work on the conda package build as a potential solution to generating the YAML templates upon installation in #347, `mpi4py` is needed during the build phase because:
- the YAML templates need to be generated
- the old loader in `httomo.data.hdf5.loaders` is used to generate the `standard_tomo` YAML template
- in `loaders.py`, there is an import of `mpi4py`

If `mpi4py` isn't needed in the import of the loader to generate its YAML template, then it can be removed from the build environment.