Remove unused function.

Author: cristianoc

cli/dune

@@ -2,5 +2,6 @@
  (name res_cli)
  (public_name rescript)
  (flags
-  (-open Syntax -open Compilerlibs406))
+  (-open Syntax -open Compilerlibs406)
+  (:standard -w +a-4-42-40-9-48))
  (libraries syntax compilerlibs406))

cli/reactjs_jsx_ppx.ml

@@ -1360,8 +1360,6 @@ module V4 = struct
     | _ -> valueStr
     [@@raises Invalid_argument]
 
-  let keyType loc = Typ.constr ~loc {loc; txt = Lident "string"} []
-
   let refType loc =
     Typ.constr ~loc
       {loc; txt = Ldot (Ldot (Lident "ReactDOM", "Ref"), "currentDomRef")}