Delete flutter_image_build.yaml (#2569)

Delete flutter_image_build.yaml
diff --git a/flutter_image_build.yaml b/flutter_image_build.yaml
deleted file mode 100644
index dc179a9..0000000
--- a/flutter_image_build.yaml
+++ /dev/null
@@ -1,10 +0,0 @@
-# Builds a new flutter image, which is used to build cocoon dashboard.
-
-steps:
-  - name: 'us-docker.pkg.dev/cloud-builders/ga/v1/docker'
-    args: ['build', '-t', 'us-docker.pkg.dev/$PROJECT_ID/flutter/flutter', '.']
-
-timeout: 1200s
-
-images: ['us-docker.pkg.dev/$PROJECT_ID/flutter/flutter']
-tags: ['cloud-builders-community']