Remove to_long and toLong, deprecated in 0.7.0
Those `Panel` methods were deprecated in December 2011, before Pandas 0.7.0 was released [1]. Probably because they used the `deprecate`decorator, they weren't spotted before, and they weren't added to #6581. So, they are still available, even if they had to be removed in Pandas 0.10.0.

1. https://github.com/pandas-dev/pandas/commit/fae6272607d4e0787a2e5476b5401144473dfced