Fix removal cross section generation in moltres_xs.py from OpenMC data

[smpark7]

Since OpenMC does not directly provide removal cross section (REMXS) tallies, moltres_xs.py calculates REMXS by adding the absorption and off-diagonal scatter cross sections for each energy group. However, the scatter cross sections include neutron multiplicity effects from (n,xn) reactions, leading to greater out-scattering than intended. Therefore, the REMXS values are smaller than intended. Conversely, neutron multiplicity from in-group scattering should be subtracted from REMXS.

This issue can be closed when the REMXS calculations in moltres_xs.py correctly exclude neutron multiplicity for neutron removal via out-scattering.