mark telemetry.agentless_enabled setting as having private visibility

lib/datadog/core/configuration/settings.rb

@@ -666,6 +666,7 @@ def initialize(*_)
           # Enable agentless mode for telemetry: submit telemetry events directly to the intake without Datadog Agent.
           #
           # @return [Boolean]
+          # @!visibility private
           option :agentless_enabled do |o|
             o.type :bool
             o.default false