Remove configuration reference from variant feature flag

[zhiyuanliang-ms]

Before stablizing 4.x, we want to drop configuration reference in variant defnition.
When we designed the configuration reference, we didn't consider other language like python and javascript. The configuration reference doesn't work well in python and javascript.

Related PR:
schema update: Azure/AppConfiguration#955
doc update: https://github.com/MicrosoftDocs/azure-docs-pr/pull/285384