Remove unused `Namespace::enclosing_record_id` field from `ir.rs`.

Before and after this CL, `enclosing_record_id` is a property of
TypeAlias:

1. Declared as a field of `struct TypeAlias` in `ir.h` and `ir.rs`
2. Read in `TypeAlias::ToJson` in `ir.cc` and in 3 places in
   `src_code_gen.rs`.
3. Written to in `importers/typedef_name.cc`

Before this CL, `enclosing_record_id` would also accidentally
be declared as a field of `struct Namespace` in `ir.rs`, even
though 1) it wasn't declared as a field in `ir.h`, and 2) wasn't
read-from or written-to anywhere.

PiperOrigin-RevId: 475014974
diff --git a/rs_bindings_from_cc/ir.rs b/rs_bindings_from_cc/ir.rs
index 99c49c4..f86451d 100644
--- a/rs_bindings_from_cc/ir.rs
+++ b/rs_bindings_from_cc/ir.rs
@@ -521,7 +521,6 @@
     pub owning_target: BazelLabel,
     #[serde(default)]
     pub child_item_ids: Vec<ItemId>,
-    pub enclosing_record_id: Option<ItemId>,
     pub enclosing_namespace_id: Option<ItemId>,
     pub is_inline: bool,
 }
diff --git a/rs_bindings_from_cc/ir_from_cc_test.rs b/rs_bindings_from_cc/ir_from_cc_test.rs
index b05e88e..a00bac3 100644
--- a/rs_bindings_from_cc/ir_from_cc_test.rs
+++ b/rs_bindings_from_cc/ir_from_cc_test.rs
@@ -2394,7 +2394,6 @@
                 name: "MyNamespace" ...
                 id: ItemId(#ns_id) ...
                 child_item_ids: [ItemId(#child_id)] ...
-                enclosing_record_id: None ...
                 enclosing_namespace_id: None ...
             }),
             IncompleteRecord(IncompleteRecord {