remove duplicate .flattened-pom.xml from .gitignore (#5897)
remove duplicate .flattened-pom.xml from .gitignore (#5897)


bpzhang/dubbo,lovepoem/dubbo,wuwen5/dubbo,yuyijq/dubbo,lovepoem/dubbo,lovepoem/dubbo,wuwen5/dubbo,wuwen5/dubbo,fengyie007/dubbo,fengyie007/dubbo,bpzhang/dubbo,yuyijq/dubbo,fengyie007/dubbo,bpzhang/dubbo