Remove preview notice from vertexai readme (#6646)

The SDK was released as GA last year.

Author: rlazo

firebase-vertexai/README.md

@@ -1,9 +1,5 @@
 # Firebase Vertex AI SDK
 
-**Preview**: Vertex AI for Firebase is in Public Preview, which means that the product is
-not subject to any SLA or deprecation policy and could change in backwards-incompatible
-ways.
-
 For developer documentation, please visit https://firebase.google.com/docs/vertex-ai.
 This README is for contributors building and running tests for the SDK.