[GHC] #5350: Validate error: unneeded import

[GHC]

#5350: Validate error: unneeded import
----------------------------------+-----------------------------------------
    Reporter:  daniel.is.fischer  |       Owner:              
        Type:  bug                |      Status:  new         
    Priority:  normal             |   Component:  Build System
     Version:  7.1                |    Keywords:              
    Testcase:                     |   Blockedby:              
          Os:  Unknown/Multiple   |    Blocking:              
Architecture:  Unknown/Multiple   |     Failure:  None/Unknown
----------------------------------+-----------------------------------------
 The import of `GHC.Show` in `IOBase.hs` is no longer needed, hence a
 validate build fails due to `-Werror`.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5350>
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