[firebase-release] Removed change log and reset repo after 15.9.0 release

Author: google-oss-bot

CHANGELOG.md

@@ -1,7 +0,0 @@
-- Added `*_EMULATOR_VERSION` env variables to allow overriding specific versions of downloadable emulators
-- Updated the functions.config deprecation notice from March 2026 to March 2027 (#9941)
-- Detects when App Hosting fails to deploy, returning an error. (#8866)
-- Add `firestore_query_collection` tool back to MCP as it is not available in the OneMCP server yet.
-- Add support for custom resolvers in Firebase Data Connect (#9967)
-- Updated the Firebase Data Connect local toolkit to v3.2.1, which includes the following changes: (#10022)
-  - Resolved metadata pathing issue for @redact and @check directives