fix unused variable

[panda-sheep]

What type of PR is this?

• bug
• feature
• enhancement

What does this PR do?

fix compilation error

[ 44%] Building CXX object src/graph/optimizer/CMakeFiles/optimizer_obj.dir/OptGroup.cpp.o
In file included from /home/eric.fang/nebula-ent/src/storage/exec/IndexEdgeScanNode.h:8:0,
from /home/eric.fang/nebula-ent/src/storage/index/LookupProcessor.cpp:15:
/home/eric.fang/nebula-ent/src/storage/exec/IndexScanNode.h: In lambda function:
/home/eric.fang/nebula-ent/src/storage/exec/IndexScanNode.h:261:25: error: unused variable 'iter' [-Werror=unused-variable]
auto [iter, result] = suffixSet.insert(std::move(suffix));
^
[ 45%] Building CXX object src/graph/executor/CMakeFiles/executor_obj.dir/query/DedupExecutor.cpp.o
[ 45%] Building CXX object src/storage/CMakeFiles/storage_admin_service_handler.dir/admin/RebuildEdgeIndexTask.cpp.o
cc1plus: all warnings being treated as errors
make[2]: *** [src/storage/CMakeFiles/graph_storage_service_handler.dir/index/LookupProcessor.cpp.o] 错误 1
make[1]: *** [src/storage/CMakeFiles/graph_storage_service_handler.dir/all] 错误 2
make[1]: *** 正在等待未完成的任务....

Which issue(s)/PR(s) this PR relates to?

Special notes for your reviewer, ex. impact of this fix, etc:

Additional context:

Checklist：

• Documentation affected （Please add the label if documentation needs to be modified.)
• Incompatible （If it is incompatible, please describe it and add corresponding label.）
• Need to cherry-pick （If need to cherry-pick to some branches, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes：

Please confirm whether to reflect in release notes and how to describe:

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