clippy: avoid manual implementation of ok

```
error: manual implementation of `ok`
    --> lightning/src/ln/channel.rs:8551:3
     |
8551 | /         match self.sign_channel_announcement(node_signer, announcement) {
8552 | |             Ok(res) => Some(res),
8553 | |             Err(_) => None,
8554 | |         }
     | |_________^ help: replace with: `self.sign_channel_announcement(node_signer, announcement).ok()`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_ok_err
     = note: `-D clippy::manual-ok-err` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(clippy::manual_ok_err)]`

```

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

lightning/src/ln/channel.rs

@@ -8548,10 +8548,7 @@ impl<SP: Deref> FundedChannel<SP> where
 			Ok(res) => res,
 			Err(_) => return None,
 		};
-		match self.sign_channel_announcement(node_signer, announcement) {
-			Ok(res) => Some(res),
-			Err(_) => None,
-		}
+		self.sign_channel_announcement(node_signer, announcement).ok()
 	}
 
 	/// May panic if called on a channel that wasn't immediately-previously