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.
Prevolutions are looped in moveset validation, so removing them should decrease the time spent on it a bit. However, this really doesn't affect the actual result in Gen 3, as the sources for moves introduced in Gen4+ are all beyond Gen 3.

In Gen 1, nevertheless, pokemon with Gen 1 moves that can only be learnt in Gen 2 as egg moves could theoretically be traded to Gen 1 via Time Capsule. This may lead to some move incompatibilities in Gen 1 which need accurate data on prevolutions in order to be accounted for.