Remove mpi4py dependency from conda package build config

[yousefmoazzam]

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.