Remove variant feature flag's configuration reference usage telemetry

[zhiyuanliang-ms]

Before stablize Microsoft.FeatureManagement 4.x, we want to drop the configuration_reference from variant feature flag because it doesn't work well with languages like python and JS.

schema change: Azure/AppConfiguration#955
doc update: https://github.com/MicrosoftDocs/azure-docs-pr/pull/285384
FM lib change: microsoft/FeatureManagement-Dotnet#488