Bugfix in TDDFPT: stop_lr is used by turbo_lanczos, turbo_davidson,
and turbo_eels. turbo_eels performs NSCF calculations, this is why we intoduced calls to mp_start_diag, set_mpi_comm_4_solvers, and unset_mpi_comm_4_solvers. Instead, turbo_lanczos, turbo_davidson do not perform NSCF, so these calls are not needed. Since the call to unset_mpi_comm_4_solvers is inside of stop_lr, this was breaking turbo_lanczos and turbo_davidson.