Clean up jsx ppx (just a refactor) (#6035)

* Clean up jsx ppx: unused mapper in signature.

* Clean up check for multiple react components.

* Rename items.

* Clean up toplevel processing of bidnings.

* Move out modifiedBinding

* Move out vbMatch

* Move out mapBinding

* Cleanup: mapBinding returns a single binding.

Author: cristianoc

res_syntax/src/reactjs_jsx_ppx.ml

@@ -108,8 +108,8 @@ let getMapper ~config =
           | Psig_attribute attr -> processConfigAttribute attr config
           | _ -> ());
           let item = default_mapper.signature_item mapper item in
-          if config.version = 3 then transformSignatureItem3 mapper item
-          else if config.version = 4 then transformSignatureItem4 mapper item
+          if config.version = 3 then transformSignatureItem3 item
+          else if config.version = 4 then transformSignatureItem4 item
           else [item])
         items
       |> List.flatten

res_syntax/src/reactjs_jsx_v3.ml

@@ -1019,7 +1019,7 @@ let jsxMapper ~config =
     | _ -> [item]
   in
 
-  let transformSignatureItem _mapper item =
+  let transformSignatureItem item =
     match item with
     | {
         psig_loc;