Configuration hashes defined by `default_source.rb` now support an
additional `:allowlist` key set equal to an array of valid values.

If an invalid value is specified for a parameter that has an allowlist,
a warning will be logged that contains the allowlist and the default
value for the parameter will be used.

::NewRelic -> NewRelic

Remove the class specific config validation that is now redundant with
the config's own allowlist validation

Now that the config handles the allowlist validation, remove the test
expecting the 8T class to do so