Revert accidental wip/generics-propeq-conservative merge

This commit reverts the commits

   e12a6a8 Propositional equality for Datatype meta-information
   0a8e6fc Make constructor metadata parametrized (with intended parameter <- datatype)
   f097b77 Implement sameConstructor
   cc618e6 get roles right and fix a FIXME
   79c7125 Actually parametrize the Constructor with the Datatype
   7bd4bab Supply a reasonable name (should be derived from d_name tho)
   09fcd70 Use 'd_name' as the name (should be derived from d_name tho)
   4d90e44 Add default case (fixes -Werror)

and effectively resets ghc.git to the state it was at commit
8c79dcb4dc2c6b8b663fa0c2e61d40d0ac0e9996