Clean-up Pokedex data for old gens

- For Gen 2 and 3, Pokedex was filled with redundant data. Gen 4 was about the same, except for Rotom formes.
- Remove references to baby prevolutions in gens where they don't exist.
- Apply https://github.com/Zarel/Pokemon-Showdown/commit/417907072cd7a55f61cf0238fe58818ea9ecc23c to Gen 1.
Also, while I am at it, we should add a bunch of <code>null</code> for unexistent mons in old gens in order to save both resources, right?