fix(es/es2015): Remove needless unreachable! (#9021)

**Related issue:**

 - Closes #9020

crates/swc/tests/fixture/issues-9xxx/9020/input/.swcrc

@@ -0,0 +1,19 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "ecmascript",
+            "jsx": false
+        },
+        "target": "es5",
+        "loose": false,
+        "minify": {
+            "compress": false,
+            "mangle": false
+        }
+    },
+    "module": {
+        "type": "es6"
+    },
+    "minify": false,
+    "isModule": true
+}

crates/swc/tests/fixture/issues-9xxx/9020/input/1.js

@@ -0,0 +1,4 @@
+const student = {
+    name: "Tom",
+    age = 12
+};

crates/swc/tests/fixture/issues-9xxx/9020/output/1.js

@@ -0,0 +1,4 @@
+var student = {
+    name: "Tom",
+    age=12
+};

crates/swc_ecma_compat_es2015/src/duplicate_keys.rs

@@ -64,7 +64,7 @@ impl VisitMut for PropFolder {
                 }
             }
 
-            Prop::Assign(..) => unreachable!("assign property in object literal is invalid"),
+            Prop::Assign(..) => {}
 
             Prop::Getter(..) => prop.visit_mut_children_with(&mut PropNameFolder {
                 props: &mut self.getter_props,