Remove support for Haddock versions < 2.0

Dropping this support is unlikely to be a problem in practice.  Debian
oldstable is currently on version 2.6.0 of Haddock, for example.

This change enables future code simplification.  Currently we
preprocess both Haskell files requiring the CPP and Literate Haskell
files; newer versions of Haddock can handle these natively.

Fixes issue #1718.

Conflicts:
	Cabal/Distribution/Simple/Haddock.hs
	Cabal/Distribution/Simple/PreProcess.hs
	Cabal/Distribution/Simple/Setup.hs