Switch release action to 'main' (#4649)

Part of switching default from `master` to `main`
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index a64acf7..1bf50a0 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -2,7 +2,7 @@
 on:
   push:
     branches:
-      - master
+      - main
 
 jobs:
   release:
@@ -49,6 +49,3 @@
         git config --global user.email ${{ secrets.USER_EMAIL }}
         dart ./script/tool/lib/src/main.dart publish-plugin --all-changed --base-sha=HEAD~ --skip-confirmation --remote=origin
       env: {PUB_CREDENTIALS: "${{ secrets.PUB_CREDENTIALS }}"}
-
-    env:
-      DEFAULT_BRANCH: master