[ntuple] remove unused kReference field type

Author: jblomer

tree/ntuple/v7/doc/specifications.md

@@ -343,9 +343,8 @@ The structural role of the field can have on of the following values
 | 0x00     | Leaf field in the schema tree                                            |
 | 0x01     | The field is the mother of a collection (e.g., a vector)                 |
 | 0x02     | The field is the mother of a record (e.g., a struct)                     |
-| 0x03     | The field is the mother of a variant (e.g., a union)                     |
-| 0x04     | The field is a reference (pointer), TODO                                 |
-| 0x05     | The field represents an unsplit object serialized with the ROOT streamer |
+| 0x03     | The field is the mother of a variant                                     |
+| 0x04     | The field represents an unsplit object serialized with the ROOT streamer |
 
 
 #### Column Description

tree/ntuple/v7/inc/ROOT/RNTupleUtil.hxx

@@ -40,7 +40,6 @@ enum ENTupleStructure {
    kCollection,
    kRecord,
    kVariant,
-   kReference, // unimplemented so far
    kUnsplit,
    kInvalid,
 };

tree/ntuple/v7/src/RNTupleSerialize.cxx

@@ -582,8 +582,7 @@ std::uint16_t ROOT::Experimental::Internal::RNTupleSerializer::SerializeFieldStr
    case ENTupleStructure::kCollection: return SerializeUInt16(0x01, buffer);
    case ENTupleStructure::kRecord: return SerializeUInt16(0x02, buffer);
    case ENTupleStructure::kVariant: return SerializeUInt16(0x03, buffer);
-   case ENTupleStructure::kReference: return SerializeUInt16(0x04, buffer);
-   case ENTupleStructure::kUnsplit: return SerializeUInt16(0x05, buffer);
+   case ENTupleStructure::kUnsplit: return SerializeUInt16(0x04, buffer);
    default: throw RException(R__FAIL("ROOT bug: unexpected field structure type"));
    }
 }
@@ -600,8 +599,7 @@ RResult<std::uint16_t> ROOT::Experimental::Internal::RNTupleSerializer::Deserial
    case 0x01: structure = ENTupleStructure::kCollection; break;
    case 0x02: structure = ENTupleStructure::kRecord; break;
    case 0x03: structure = ENTupleStructure::kVariant; break;
-   case 0x04: structure = ENTupleStructure::kReference; break;
-   case 0x05: structure = ENTupleStructure::kUnsplit; break;
+   case 0x04: structure = ENTupleStructure::kUnsplit; break;
    default: return R__FAIL("unexpected on-disk field structure value");
    }
    return result;