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/primitives/runtime/src/generic/mod.rs

@@ -29,9 +29,9 @@ mod unchecked_extrinsic;
 
 pub use self::{
 	block::{Block, BlockId, SignedBlock},
-	checked_extrinsic::{CheckedExtrinsic, ExtrinsicFormat},
+	checked_extrinsic::CheckedExtrinsic,
 	digest::{Digest, DigestItem, DigestItemRef, OpaqueDigestItemId},
 	era::{Era, Phase},
 	header::Header,
-	unchecked_extrinsic::{Preamble, SignedPayload, UncheckedExtrinsic},
+	unchecked_extrinsic::{SignedPayload, UncheckedExtrinsic},
 };