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Remove unused known keys and names for generics classes

Wolfgang Jeltsch requested to merge wip/jeltsch/known-key-removals/generics-classes into master

This removes the known-key and corresponding name variables for Datatype, Constructor, and Selector from Data.Generics, as they are apparently nowhere used in GHC’s source code, and thus resolves part of #26677 (closed).

I could build GHC successfully with this change. I fed to the resulting GHCi a simple data type declaration with Generic deriving, which didn’t cause any apparent problems.

However, when combining the changes in this merge request with those in !15184 (closed), building the stage-2 GHC fails with error messages like the following one:

ghc: panic! (the 'impossible' happened)
  GHC version 9.15.20251216:
	getSymtabName:unknown known-key unique
  2361393446
  34K
  3
  294
  Call stack:
      CallStack (from HasCallStack):
        pprPanic, called at compiler/GHC/Iface/Binary.hs:745:34 in ghc-9.15-inplace:GHC.Iface.Binary

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

To me this seems as if the changes in this merge request or the ones in !15184 (closed) are incorrect. I would appreciate help by the experts here. 🙂

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