Revert "Add code to convert from representation types, to existentially quantified singletons."

This reverts commit f7fb908ad963f7180c30b55fba57a858b0391de4.

It causes a valdiate failure:

*** Core Lint errors : in result of Simplifier ***
<no location info>: Warning:
    In the expression: f_aqM
                         (x_aqF
                          `cast` (GHC.TypeLits.NTCo:SingI[0] <k_aKs> <n_aKt>
                                  :: GHC.TypeLits.SingI k_aKs n_aKt
                                       ~#
                                     GHC.TypeLits.Sing k_aKs n_aKt))
    From-type of Cast differs from type of enclosed expression
    From-type: GHC.TypeLits.SingI k_aKs n_aKt
    Type of enclosed expr: GHC.TypeLits.Sing k_aHe n_aHf
    Actual enclosed expr: x_aqF
    Coercion used in cast: GHC.TypeLits.NTCo:SingI[0] <k_aKs> <n_aKt>
*** Offending Program ***
[...]