Address cargo run warnings

[kishorbhat]

$ cargo run
   Compiling gmail_stats v0.1.0 (<filepath>)
warning: unused import: `tokio::task`
  --> src/main.rs:10:5
   |
10 | use tokio::task;
   |     ^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: variable does not need to be mutable
  --> src/main.rs:69:9
   |
69 |     let mut retries = 0;
   |         ----^^^^^^^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default