Remove to_long and toLong, deprecated in 0.7.0

[datapythonista]

Those Panel methods were deprecated in December 2011, before Pandas 0.7.0 was released [1]. Probably because they used the deprecatedecorator, 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. fae6272