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/grandpa/src/mock.rs

@@ -35,8 +35,11 @@ use sp_consensus_grandpa::{RoundNumber, SetId, GRANDPA_ENGINE_ID};
 use sp_core::{crypto::KeyTypeId, H256};
 use sp_keyring::Ed25519Keyring;
 use sp_runtime::{
-	curve::PiecewiseLinear, generic::UncheckedExtrinsic, impl_opaque_keys,
-	testing::UintAuthorityId, traits::OpaqueKeys, BuildStorage, DigestItem, Perbill,
+	curve::PiecewiseLinear,
+	impl_opaque_keys,
+	testing::{TestXt, UintAuthorityId},
+	traits::OpaqueKeys,
+	BuildStorage, DigestItem, Perbill,
 };
 use sp_staking::{EraIndex, SessionIndex};
 
@@ -74,7 +77,7 @@ where
 	RuntimeCall: From<C>,
 {
 	type OverarchingCall = RuntimeCall;
-	type Extrinsic = UncheckedExtrinsic<u64, RuntimeCall, (), ()>;
+	type Extrinsic = TestXt<RuntimeCall, ()>;
 }
 
 parameter_types! {