Remove slow / redundant test for RecipientCSV
Remove slow / redundant test for RecipientCSV

"test_processing_a_big_list" checks some work was done, but this
is evident in the results of other tests. It looks like added it
originally to understand performance [1], which is still an issue
but we'll address it in a different way in later commits.

[1]: https://github.com/alphagov/notifications-utils/commit/03775dab920d0c251d2eabe649ba36247db19d4b

alphagov/notifications-utils