Remove Shake database on clean.
Remove Shake database on clean.

See #131. [skip ci]

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,izgzhen/hadrian,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,bgamari/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc