Fix Sonic Screwdriver Warnings

Currently, we get these warnings when compiling the Rust code.

warning: unused import: `OurSmtpTransport`
  --> application/src/adapters/mod.rs:17:32
   |
17 | pub use email::{EmailsAdapter, OurSmtpTransport};
   |                                ^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: field `0` is never read
   --> http/src/web/auth/oauth.rs:116:18
    |
116 |     RequestError(BasicRequestTokenError<oauth2::reqwest::Error<::reqwest::Error>>),
    |     ------------ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |     |
    |     field in this variant
    |
    = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
116 |     RequestError(()),
    |                  ~~

These should be fixed.