agent: Remove useless loop

This is the report from `make check`:

```
error: this loop never actually loops
   --> src/signal.rs:147:9
    |
147 | /         loop {
148 | |             select! {
149 | |                 _ = handle => {
150 | |                     println!("INFO: task completed");
...   |
156 | |             }
157 | |         }
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#never_loop
    = note: `#[deny(clippy::never_loop)]` on by default
```

There is only one option: you get something or a timeout. You never retry, so
the report is correct.

Fixes: #9342

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

src/agent/src/signal.rs

@@ -144,11 +144,9 @@ mod tests {
 
         tx.send(true).expect("failed to request shutdown");
 
-        loop {
         select! {
             _ = handle => {
                 println!("INFO: task completed");
-                    break;
             },
             _ = &mut timeout => {
                 panic!("signal thread failed to stop");
@@ -156,4 +154,3 @@ mod tests {
         }
     }
 }
-}