fix: remove unused imports in memtable_util.rs

[v0y4g3r]

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What's changed and what's your intention?

remove unused imports in memtable_util.rs. Clippy does not deny these unused imports as we explicitly allow this.

greptimedb/src/mito2/src/lib.rs

Line 23 in f81e37f

#[cfg_attr(feature = "test", allow(unused))]

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.21%. Comparing base (8059b95) to head (d574a63).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3376      +/-   ##
==========================================
- Coverage   85.61%   85.21%   -0.41%     
==========================================
  Files         893      893              
  Lines      147210   147210              
==========================================
- Hits       126039   125438     -601     
- Misses      21171    21772     +601