[GHC] #13561: Remove unsafe Chan combinators

[GHC]

#13561: Remove unsafe Chan combinators
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  high           |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #4154
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Namely `isEmptyChan` and `unGetChan`. See #4154. Let's delete them for
 8.4.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13561>
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