Clear changelog after v3.0.0 release

Author: thechenky

changelog.txt

@@ -1,5 +0,0 @@
-important - [breaking change] The Firebase Functions SDK no longer supports Node 6. Developers must use Node 8.13.0 or higher.
-important - [breaking change] The Firebase Functions SDK no longer supports Firebase Tools SDK below version 6.8.0.
-important - [breaking change] FIREBASE_PROJECT environment variable is no longer supported and has been supplanted by FIREBASE_CONFIG.
-fixed - Fixed bug in Node 10 where events from Node 10 were not parsed properly resulting in undefined.
-fixed - Set GCLOUD_PROJECT from FIREBASE_CONFIG when missing.