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/minimal/runtime/src/lib.rs

@@ -54,7 +54,7 @@ pub fn native_version() -> NativeVersion {
 	NativeVersion { runtime_version: VERSION, can_author_with: Default::default() }
 }
 
-type TxExtension = (
+type SignedExtra = (
 	frame_system::CheckNonZeroSender<Runtime>,
 	frame_system::CheckSpecVersion<Runtime>,
 	frame_system::CheckTxVersion<Runtime>,
@@ -111,7 +111,7 @@ impl pallet_transaction_payment::Config for Runtime {
 
 impl pallet_minimal_template::Config for Runtime {}
 
-type Block = frame::runtime::types_common::BlockOf<Runtime, TxExtension>;
+type Block = frame::runtime::types_common::BlockOf<Runtime, SignedExtra>;
 type Header = HeaderFor<Runtime>;
 
 type RuntimeExecutive =