Update mapping for new Schwab CSV format

[tstabrawa]

Unfortunately, within a couple months of #117 being merged, Schwab changed their CSV format again. On the bright side, it's much easier to parse, as it doesn't have extra lines before and after the header. On the down side, the account ID is no longer in the CSV content, so we can't parse that out anymore.

For what it's worth, the account ID is still provided in the CSV filename, but this PR doesn't attempt to extract it from there.

Commits:

• Update mapping for new Schwab CSV format
  • Remove account ID parsing, custom_header checking, skipping extra lines, etc. Tweak field names. Update tests.

All tests pass locally. I've also been successfully using this to parse downloaded CSV files for several months.