style: Remove unused structs

`cargo clippy` reports:

```
error: struct `StoredObjectField` is never constructed
  --> graphql_client_codegen/src/schema.rs:14:8
   |
14 | struct StoredObjectField {
   |        ^^^^^^^^^^^^^^^^^
   |
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

error: struct `ObjectFieldId` is never constructed
  --> graphql_client_codegen/src/schema.rs:51:19
   |
51 | pub(crate) struct ObjectFieldId(usize);
   |                   ^^^^^^^^^^^^^

error: struct `InputFieldId` is never constructed
  --> graphql_client_codegen/src/schema.rs:72:8
   |
72 | struct InputFieldId(usize);
   |        ^^^^^^^^^^^^
```

Author: caspermeijn

graphql_client_codegen/src/schema.rs

@@ -10,12 +10,6 @@ use std::collections::{BTreeMap, BTreeSet};
 
 pub(crate) const DEFAULT_SCALARS: &[&str] = &["ID", "String", "Int", "Float", "Boolean"];
 
-#[derive(Debug, PartialEq, Clone)]
-struct StoredObjectField {
-    name: String,
-    object: ObjectId,
-}
-
 #[derive(Debug, PartialEq, Clone)]
 pub(crate) struct StoredObject {
     pub(crate) name: String,
@@ -47,9 +41,6 @@ pub(crate) enum StoredFieldParent {
 #[derive(Debug, Clone, Copy, PartialEq, Hash, Eq, PartialOrd, Ord)]
 pub(crate) struct ObjectId(u32);
 
-#[derive(Debug, Clone, Copy, PartialEq, Hash, Eq)]
-pub(crate) struct ObjectFieldId(usize);
-
 #[derive(Debug, Clone, Copy, PartialEq, Hash, Eq, PartialOrd, Ord)]
 pub(crate) struct InterfaceId(usize);
 
@@ -68,9 +59,6 @@ pub(crate) struct InputId(u32);
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub(crate) struct StoredFieldId(usize);
 
-#[derive(Debug, Clone, Copy, PartialEq)]
-struct InputFieldId(usize);
-
 #[derive(Debug, Clone, PartialEq)]
 pub(crate) struct StoredInterface {
     name: String,