The symptom is that I can specify mechanism=MONGODB-CR when creating a new user without getting any error message, but later I cannot auth using MONGODB-CR for that new user. Confusing.
Since this field does not make sense in a 2.8+ world, we should just remove it.

Reference:
https://github.com/mongodb/mongo/blob/master/src/mongo/db/auth/user_management_commands_parser.cpp#L243
https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/user_management_commands.cpp#L424-L427