[GHC] #8719: clarify prefetch release notes + remove some deadcode

[GHC]

#8719: clarify prefetch release notes + remove some deadcode
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       Reporter:  carter            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:  7.8.1
      Component:  Compiler          |           Version:  7.8.1-rc1
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 https://github.com/cartazio/ghc/compare/ghc:ghc-7.8...amend-prefetch-
 release-notes

 noticed some deadcode in the -fvia-c related to prefetch, removes that too

 i have those two as separate commits so they can be handled more easily

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8719>
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