Do not depend on hspec-discover (the haskell library), as that is empty
anyways. We ensure that the hspec-discover binary is available via
control/debian.

--- a/hspec.cabal
+++ b/hspec.cabal
@@ -47,7 +47,6 @@
       QuickCheck >=2.12
     , base ==4.*
     , hspec-core ==2.6.1
-    , hspec-discover ==2.6.1
     , hspec-expectations ==0.8.2.*
   exposed-modules:
       Test.Hspec
