check if validation under kExtended src/mongo/bson/bson_validate.cpp is even required.

Currently, it seems like we're doing a redundant check here since we already validate prior to this (see: validate_adaptor.cpp, where mongod --validate calls validateBSON() on each record, getBSONValidateMode() returns kFull when BSON conformance validation is enabled, otherwise kExtended, and for BinDataType::Column, the V2 (validation version 2) path already does BSONColumn structural validation during the main BSON byte walk, before checkNonConformantElem() runs.)