USER-DPD: propagate a minor performance bugfix throughout the DPDE code

The fix_shardlow_kokkos.cpp code had already factored out a redundant
sqrt() calculation in the innermost loop of ssa_update_dpde().  This
changeset propagates an equivilent optimization to:
  fix_shardlow.cpp
  pair_dpd_fdt_energy.cpp
  pair_dpd_fdt_energy_kokkos.cpp
The alpha_ij variable was really just an [itype][jtype] lookup parameter,
replacing a sqrt() and two multiplies per interacting particle pair
by a cached memory read.  Even if there isn't much time savings, the
code is now consistent across the various versions.