fix: remove unused imports in memtable_util.rs (#3376) #113

Job	Run time
Clippy	8m 14s
Build GreptimeDB binaries (ubuntu-20.04)	12m 41s
coverage	1s
Check (windows-latest)	12m 6s
Rustfmt	45s
Check (ubuntu-20.04)	7m 54s
Toml Check	35s
Spell Check with Typos	16s
Sqlness Test (ubuntu-20.04)	1m 42s
Sqlness Test with Kafka Wal (ubuntu-20.04)	2m 19s
Compatibility Test	27s
	47m 0s

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