Driver: Fix scheduler crash after task closure

A removed audio task could still have one or more driver messages left in its queue, leading to a crash when the id->mixer lookup failed. This removes an unwrap which is invalid under these assumptions and includes an extra cleanup measure for message forwarders under the same circumstances.

This was tested using `cargo make ready`.

src/input/sources/http.rs

@@ -13,7 +13,6 @@ use reqwest::{
     header::{HeaderMap, ACCEPT_RANGES, CONTENT_LENGTH, CONTENT_TYPE, RANGE, RETRY_AFTER},
     Client,
 };
-use std::fmt::format;
 use std::{
     io::{Error as IoError, ErrorKind as IoErrorKind, Result as IoResult, SeekFrom},
     pin::Pin,