test(spin-mpi): drop committed deeppot_dpa3_spin_mpi.yaml

gen_spin.py rebuilds the yaml from the deterministic config + seed in
_build_dpa3_mpi_yaml when missing, and the multi-rank test compares
mpi-2 vs same-archive mpi-1 (no hardcoded numerical references depend
on these weights). Unlike deeppot_dpa_spin.yaml -- whose committed
weights are pinned because C++ tests hardcode reference values against
them -- nothing requires deeppot_dpa3_spin_mpi.yaml to be checkpointed
in git.

Author: Han Wang