fix: remove unsafe impl Sync trait for LruCacheShard

[MrCroxx]

What's changed and what's your intention?

LruCacheShard only need to be Send for the async environment. It's not thread-safe and is protected by Mutex in LruCache.

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #2996 (438feb0) into main (d7e6420) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2996      +/-   ##
==========================================
- Coverage   72.98%   72.98%   -0.01%     
==========================================
  Files         723      723              
  Lines       96957    96957              
==========================================
- Hits        70766    70765       -1     
- Misses      26191    26192       +1     

Flag	Coverage Δ
rust	72.98% <ø> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/common/src/cache.rs	96.61% <ø> (ø)
src/meta/src/barrier/mod.rs	65.27% <0.00%> (-0.33%)	⬇️

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[wenym1]

LGTM