Revert "FRAME: Create TransactionExtension as a replacement for SignedExtension (#2280)" (#3665)

This PR reverts #2280 which introduced `TransactionExtension` to replace
`SignedExtension`.

As a result of the discussion
[here](https://github.com/paritytech/polkadot-sdk/pull/3623#issuecomment-1986789700),
the changes will be reverted for now with plans to reintroduce the
concept in the future.

---------

Signed-off-by: georgepisaltu <george.pisaltu@parity.io>

templates/solochain/node/src/benchmarking.rs

@@ -109,7 +109,7 @@ pub fn create_benchmark_extrinsic(
 		.checked_next_power_of_two()
 		.map(|c| c / 2)
 		.unwrap_or(2) as u64;
-	let tx_ext: runtime::TxExtension = (
+	let extra: runtime::SignedExtra = (
 		frame_system::CheckNonZeroSender::<runtime::Runtime>::new(),
 		frame_system::CheckSpecVersion::<runtime::Runtime>::new(),
 		frame_system::CheckTxVersion::<runtime::Runtime>::new(),
@@ -121,12 +121,11 @@ pub fn create_benchmark_extrinsic(
 		frame_system::CheckNonce::<runtime::Runtime>::from(nonce),
 		frame_system::CheckWeight::<runtime::Runtime>::new(),
 		pallet_transaction_payment::ChargeTransactionPayment::<runtime::Runtime>::from(0),
-	)
-		.into();
+	);
 
 	let raw_payload = runtime::SignedPayload::from_raw(
 		call.clone(),
-		tx_ext.clone(),
+		extra.clone(),
 		(
 			(),
 			runtime::VERSION.spec_version,
@@ -144,7 +143,7 @@ pub fn create_benchmark_extrinsic(
 		call,
 		sp_runtime::AccountId32::from(sender.public()).into(),
 		runtime::Signature::Sr25519(signature),
-		tx_ext,
+		extra,
 	)
 }