[turbofan] Remove the output_index field that was unused in Node::Use.

BUG=

Review URL: https://codereview.chromium.org/1294913003

Cr-Commit-Position: refs/heads/master@{#30212}

src/compiler/node.h

@@ -210,7 +210,6 @@ class Node final {
     uint32_t bit_field_;
 
     int input_index() const { return InputIndexField::decode(bit_field_); }
-    int output_index() const { return OutputIndexField::decode(bit_field_); }
     bool is_inline_use() const { return InlineField::decode(bit_field_); }
     Node** input_ptr() {
       int index = input_index();
@@ -229,7 +228,8 @@ class Node final {
 
     typedef BitField<bool, 0, 1> InlineField;
     typedef BitField<unsigned, 1, 17> InputIndexField;
-    typedef BitField<unsigned, 17, 14> OutputIndexField;
+    // Leaving some space in the bitset in case we ever decide to record
+    // the output index.
   };
 
   //============================================================================