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/src/traits/misc.rs

@@ -919,13 +919,24 @@ pub trait ExtrinsicCall: sp_runtime::traits::Extrinsic {
 	fn call(&self) -> &Self::Call;
 }
 
+#[cfg(feature = "std")]
+impl<Call, Extra> ExtrinsicCall for sp_runtime::testing::TestXt<Call, Extra>
+where
+	Call: codec::Codec + Sync + Send + TypeInfo,
+	Extra: TypeInfo,
+{
+	fn call(&self) -> &Self::Call {
+		&self.call
+	}
+}
+
 impl<Address, Call, Signature, Extra> ExtrinsicCall
 	for sp_runtime::generic::UncheckedExtrinsic<Address, Call, Signature, Extra>
 where
 	Address: TypeInfo,
 	Call: TypeInfo,
 	Signature: TypeInfo,
-	Extra: TypeInfo,
+	Extra: sp_runtime::traits::SignedExtension + TypeInfo,
 {
 	fn call(&self) -> &Self::Call {
 		&self.function