Revert "ci: add compiler flag to avoid build error (#880)"

This reverts commit deeb0e0.

Author: Watson1978

.github/workflows/maintenance-bundle-update.yml

@@ -12,7 +12,6 @@ jobs:
     runs-on: windows-latest
     env:
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      CFLAGS: '-Wno-error=sign-conversion' # Compiler flag to avoid build error in libiconv
     steps:
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1