[giga] clear up cache after Write

[codchen]

Describe your changes and provide context

The logic to preserve cache entries was added to circumvent the visibility issue with base store. This PR removes it because:

1. it's no longer needed as add original cachekv as base layer #2780 has addressed the visibility issue
2. it causes stale cache reads if some balance is modified in giga, the tx falls back, and v2 modifies the balance to a different value, then subsequent txs will see incorrect balance when processed in giga

Testing performed to validate your change

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 10, 2026, 4:35 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.77%. Comparing base (28bc1c0) to head (7066b55).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2827      +/-   ##
==========================================
- Coverage   56.77%   56.77%   -0.01%     
==========================================
  Files        2070     2070              
  Lines      168274   168270       -4     
==========================================
- Hits        95542    95532      -10     
+ Misses      64247    64246       -1     
- Partials     8485     8492       +7     

Flag	Coverage Δ
sei-chain	40.85% <100.00%> (-0.01%)	⬇️
sei-cosmos	48.13% <ø> (+0.01%)	⬆️
sei-db	68.72% <ø> (ø)
sei-tendermint	58.70% <ø> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
giga/deps/store/cachekv.go	78.88% <100.00%> (-0.90%)	⬇️

... and 36 files with indirect coverage changes

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