Revert "Add missing translation for "purchase" [Fixes #32]"

This reverts commit ce3169aae3566db33b29084c5ec8257bf6619e07.

As pointed out by @futhr in
https://github.com/spree/spree_wishlist/commit/ce3169aae3566db33b29084c5ec8257bf6619e07#commitcomment-3344337,
this translation key is not necessary.