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.

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Signed-off-by: georgepisaltu <[email protected]>