Fix clippy warnings

src/main.rs

@@ -1,4 +1,4 @@
-#[deny(unsafe_code)]
+#![deny(unsafe_code)]
 #[macro_use]
 extern crate clap;
 #[macro_use]

src/session.rs

@@ -1,7 +1,6 @@
 use anyhow::{Context, Result};
 use directories::ProjectDirs;
 use matrix_sdk::{Client, ClientConfig, Session};
-use serde_lexpr;
 use std::fs::{create_dir_all, File};
 use std::io::ErrorKind;