We no longer define the `__HADDOCK_VERSION__` macro when invoking GHC
through Haddock, since doing so essentially guarantees recompilation
during documentation generation.

We audited all uses of `__HADDOCK_VERSION__` in hackage, ensuring there
was a reasonable path forward to migrate away from using
`__HADDOCK_VERSION__` for each, while generating the same documentation
as it did before.

If you are a user of `__HADDOCK_VERSION__`, please take a look at the
discussion in https://github.com/haskell/cabal/pull/9177 and reach out
to us if your use case is not covered.

Reverts the version-cpp flag introduced in the previous commit to avoid
a workaround. Instead, we get rid of the problem (`__HADDOCK_VERSION__`) at its root.

See the discussion in #9177