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>

substrate/frame/election-provider-multi-phase/test-staking-e2e/src/mock.rs

@@ -62,7 +62,7 @@ pub const INIT_TIMESTAMP: BlockNumber = 30_000;
 pub const BLOCK_TIME: BlockNumber = 1000;
 
 type Block = frame_system::mocking::MockBlockU32<Runtime>;
-type Extrinsic = sp_runtime::generic::UncheckedExtrinsic<u64, RuntimeCall, (), ()>;
+type Extrinsic = testing::TestXt<RuntimeCall, ()>;
 
 frame_support::construct_runtime!(
 	pub enum Runtime {
@@ -699,7 +699,7 @@ pub fn roll_to_with_ocw(n: BlockNumber, pool: Arc<RwLock<PoolState>>, delay_solu
 			for encoded in &pool.read().transactions {
 				let extrinsic = Extrinsic::decode(&mut &encoded[..]).unwrap();
 
-				let _ = match extrinsic.function {
+				let _ = match extrinsic.call {
 					RuntimeCall::ElectionProviderMultiPhase(
 						call @ Call::submit_unsigned { .. },
 					) => {