[GHC] #13521: Remove comments about API annotations

[GHC]

#13521: Remove comments about API annotations
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        Reporter:  mpickering        |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  ApiAnnotations
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13012            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 I agree and had been meaning to suggest doing this myself. Particularly
 the comments on `HsExpr` occupy a lot of prime real estate for something
 which most people working with `HsExpr` won't need to know about (I still
 don't really understand what any of this `ApiAnnotation` is about). It
 would be better to move this information somewhere else.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13521#comment:2>
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