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/support/test/tests/runtime.rs

@@ -895,7 +895,7 @@ fn test_metadata() {
 		ty: meta_type::<UncheckedExtrinsic>(),
 		version: 4,
 		signed_extensions: vec![SignedExtensionMetadata {
-			identifier: "UnitTransactionExtension",
+			identifier: "UnitSignedExtension",
 			ty: meta_type::<()>(),
 			additional_signed: meta_type::<()>(),
 		}],