Remove reference to internals in error message for unused values #501

Job	Run time
windows-msvc-shared (MSGPACK)	4m 17s
windows-msvc-shared (FLEXBUFFERS)	5m 46s
windows-msvc-shared (AVRO)	7m 14s
windows-msvc-shared (JSON)	44s
windows-msvc-shared (CBOR)	8m 24s
windows-msvc-shared (XML)	3m 50s
windows-msvc-shared (PARQUET)	40m 23s
windows-msvc-shared (UBJSON)	1m 10s
windows-msvc-shared (TOML)	4m 4s
windows-msvc-shared (CAPNPROTO)	8m 23s
windows-msvc-shared (YAML)	5m 43s
windows-msvc (XML)	4m 10s
windows-msvc (AVRO)	7m 33s
windows-msvc (CBOR)	8m 55s
windows-msvc (CAPNPROTO)	7m 49s
windows-msvc (TOML)	4m 23s
windows-msvc (JSON)	59s
windows-msvc (MSGPACK)	4m 6s
windows-msvc (UBJSON)	7m 34s
windows-msvc (PARQUET)	40m 13s
windows-msvc (FLEXBUFFERS)	5m 11s
windows-msvc (benchmarks)	47m 32s
windows-msvc (YAML)	4m 38s
	3h 53m 1s

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