elliptic-curve: remove weierstrass module (#2005)

This code has been extracted into the `primeorder` crate:

https://github.com/RustCrypto/elliptic-curves/tree/master/primeorder

Author: tarcieri

elliptic-curve/src/lib.rs

@@ -94,8 +94,6 @@ pub mod ecdh;
 pub mod ops;
 #[cfg(feature = "sec1")]
 pub mod sec1;
-#[cfg(feature = "arithmetic")]
-pub mod weierstrass;
 
 mod error;
 mod field;