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.
Wait, no, newer mons are allowed to play under old mechanic rules in Custom Game and for certain other mechanics.