Update available_ppc()

[avehtari]

available_ppc() lists also PPC functions that are deprecated. For example, the list includes

  ppc_loo_pit
  ppc_loo_pit_overlay
  ppc_loo_pit_qq

but ppc_loo_pit is deprecated. It would be nice to update the list to include only the non-deprecated PPC functions.

[jgabry]

Yeah good idea. I’ll update it.