Remove birthday store

[arthyn]

This showed up originally as an infinite loop seen at the end of an L1 transaction, basically anything using useEthereumTransaction that also has it's refetch callback depend upon either syncExtras or syncDates, which in this case was ChangeSponsor. I was originally going to just fix the issue inside birthday store:

const addToBirthdayCache = useCallback(
    entry =>
      _setBirthdayCache(cache => ({
        ...cache,
        ...entry,
      })),
    [_setBirthdayCache]
  );

  const getBirthday = useCallback(point => birthdayCache[point] || Nothing(), [
    birthdayCache,
  ]);

  const syncBirthday = useCallback(
  ...
  addToBirthdayCache({
  ...
  [contracts, web3, addToBirthdayCache, getBirthday]

You can maybe spot the issue, but it's kinda subtle. Basically the call to syncBirthday rebuilds the birthday cache every time because addToBirthdayCache creates a new object each time. Since getBirthday also has birthdayCache as a dependency and getBirthday is a dependency of syncBirthday that means that anytime syncBirthday is called, it changes itself and shows up as changed to anyone depending on it. So way down the line this causes an infinite loop. When I started looking for usages of birthday stuff I noticed we don't actually use the contents of the birthday store anywhere so I just decided to remove it since it's unnecessary.