Remove unnecessary dependency of `mpi4py` in `standard_tomo.py`

The MPI communicator was found to not be needed for parallel reads, hence there is no need for it to be there technically speaking. (Originally noted in https://github.com/DiamondLightSource/httomo/issues/208#issuecomment-1934087364)

Once #208 is done, this can be done.

Start by reviewing standard_tomo.py and the discussion in issue #208, which must be completed first. Confirm where the MPI communicator and mpi4py dependency are referenced for parallel reads; done means the unnecessary dependency is removed without affecting the intended reads.

Written by the indexing model from the issue text.