Delete some dead substitution functions (#349)

`substKindInPred` is completely unused, and moreover, its definition
is somewhat dubious, given that it only substitutes into kinds, not
types. We already have `substPred`, so the prudent thing to do seems
to be to make `substPred` substitute into kinds, just like its
counterpart `substType`.

`substKindInTvb` is also dead code, so let's also axe that.

src/Data/Singletons/Util.hs

@@ -318,25 +318,12 @@ substPred :: Map Name DType -> DPred -> DPred
 substPred subst pred | Map.null subst = pred
 substPred subst (DAppPr pred ty) =
   DAppPr (substPred subst pred) (substType subst ty)
-substPred subst (DSigPr pred ki) = DSigPr (substPred subst pred) ki
+substPred subst (DSigPr pred ki) =
+  DSigPr (substPred subst pred) (substKind subst ki)
 substPred _ pred@(DVarPr {}) = pred
 substPred _ pred@(DConPr {}) = pred
 substPred _ pred@DWildCardPr = pred
 
-substKindInPred :: Map Name DKind -> DPred -> DPred
-substKindInPred subst pred | Map.null subst = pred
-substKindInPred subst (DAppPr pred ty) =
-  DAppPr (substKindInPred subst pred) (substType subst ty)
-substKindInPred subst (DSigPr pred ki) = DSigPr (substKindInPred subst pred)
-                                                (substKind subst ki)
-substKindInPred _ pred@(DVarPr {}) = pred
-substKindInPred _ pred@(DConPr {}) = pred
-substKindInPred _ pred@DWildCardPr = pred
-
-substKindInTvb :: Map Name DKind -> DTyVarBndr -> DTyVarBndr
-substKindInTvb _ tvb@(DPlainTV _) = tvb
-substKindInTvb subst (DKindedTV n ki) = DKindedTV n (substKind subst ki)
-
 cuskify :: DTyVarBndr -> DTyVarBndr
 cuskify (DPlainTV tvname) = DKindedTV tvname $ DConT typeKindName
 cuskify tvb               = tvb