Remove unused 'src/etc/ziggurat_tables.py' Python script.

This Python script was used to generate a `ziggurat_tables.rs` file in
librand, but librand was moved out of the repo.

* https://github.com/rust-lang/rust/commits/master/src/librand/distributions/ziggurat_tables.rs
* https://github.com/rust-lang-nursery/rand/blob/master/utils/ziggurat_tables.py

Author: frewsxcv