cosmrs: type parameter error SigningKey::derive_from_path

[chris-ricketts]

Unneeded type parameter in SigningKey::derive_from_path causes build error:

  --> src/account.rs:22:13
   |
22 |             SigningKey::derive_from_path(seed, &path).expect("private key derivation failed");
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for type parameter `S` declared on the associated function `derive_from_path`