fix(messaging): fix clippy/rustdoc errors in benchmarking

- Mark shell command doc block as ```text to fix rustdoc parsing
- Remove duplicated #![cfg(feature = "runtime-benchmarks")] (already gated in lib.rs)
- Use let _ = for unused MultiRemovalResults from clear_prefix

pezcumulus/teyrchains/pezpallets/messaging/src/benchmarking.rs

@@ -1,7 +1,7 @@
 //! Benchmarking setup for pezpallet-messaging
 //!
 //! Run benchmarks with:
-//! ```
+//! ```text
 //! ./target/release/frame-omni-bencher v1 benchmark pezpallet \
 //!   --runtime target/release/wbuild/people-pezkuwichain-runtime/people_pezkuwichain_runtime.compact.compressed.wasm \
 //!   --pallets pezpallet_messaging -e all --steps 50 --repeat 20 \
@@ -9,8 +9,6 @@
 //!   --template bizinikiwi/.maintain/frame-weight-template.hbs
 //! ```
 
-#![cfg(feature = "runtime-benchmarks")]
-
 use super::*;
 use pezframe_benchmarking::v2::*;
 use pezframe_system::RawOrigin;
@@ -68,7 +66,7 @@ mod benchmarks {
 
 		#[block]
 		{
-			Inbox::<T>::clear_prefix(era, n, None);
+			let _ = Inbox::<T>::clear_prefix(era, n, None);
 		}
 	}