chore: remove dead code and stale comments

- Drop dead Rooms/Room infrastructure and the rooms field on Context.
- Remove unused ActorMessage::Tick variant and the now-empty handler arm.
- Strip dead active-thread tracking machinery from ThreadPool (unused
  getters, _id/_active_threads/_total_threads parameters).
- Flatten ServerActor::initiate_connection: the duplicated retry block
  was a copy-paste retry against the same address with the same args
  and could never succeed where the first attempt failed.
- Delete commented-out print_hex2/decode debug stubs, dead test
  fixtures in login.rs and message_user.rs, the 47-line commented body
  of RoomListHandler, and assorted // TODO theater.

soulseek-rs-lib/src/utils/md5.rs

@@ -361,7 +361,7 @@ fn compute_md5_digest(mut v: Vec<u8>) -> String {
 */
 fn bit_padding(input: &str) -> Vec<u8> {
     let mut input_vector: Vec<u8> = convert_str_to_vec(input);
-    let bit_length: u64 = (input.len() as u64) * 8u64; // todo - add support for > 2^64 bit size
+    let bit_length: u64 = (input.len() as u64) * 8u64;
 
     // 128_u8 is the equivalent of padding 1 as an unsigned 8-bit integer
     // with lower-order bits first