Don't call alex for Cabal lib during GHC build

The Cabal library now commits `Lexer.hs` directly to the source tree,
so the build step where we'd call alex ourselves to generate that file
is no longer necessary, nor will it work.

See also: https://ghc.haskell.org/trac/ghc/ticket/14459

Reviewers: bgamari, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14459

Differential Revision: https://phabricator.haskell.org/D4240