[GHC] #10913: deprecate and then remove -fwarn-hi-shadowing

[GHC]

#10913: deprecate and then remove -fwarn-hi-shadowing
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              Reporter:  osa1        |             Owner:
                  Type:  task        |            Status:  new
              Priority:  lowest      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:  newcomer    |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 I realized that the flag is not used in the compiler at all, and @slyfox
 realized that the flag was last used in 2006. He also found out that there
 are 355 packages in the Hackage that use this useless flag. So we should
 maybe first deprecate it and remove it later.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10913>
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