[giga] avoid double signature decoding

[codchen]

Describe your changes and provide context

In giga signatures are explicitly decoded here https://github.com/sei-protocol/sei-chain/blob/main/app/app.go#L1723 so the previous decoding (that's skipped by this PR) is not necessary. For the fallback scenario, signature decoding will be handled by the ante handler's lazy decoding logic: https://github.com/sei-protocol/sei-chain/blob/main/x/evm/ante/preprocess.go#L59

Testing performed to validate your change

existing tests

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Feb 5, 2026, 3:38 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.64%. Comparing base (3d6356d) to head (cde2212).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2797      +/-   ##
==========================================
+ Coverage   46.97%   56.64%   +9.66%     
==========================================
  Files        1965     2031      +66     
  Lines      160525   165854    +5329     
==========================================
+ Hits        75410    93940   +18530     
+ Misses      78585    63681   -14904     
- Partials     6530     8233    +1703     

Flag	Coverage Δ
sei-chain	41.50% <100.00%> (-0.05%)	⬇️
sei-cosmos	48.12% <ø> (-0.01%)	⬇️
sei-db	68.72% <ø> (ø)
sei-tendermint	58.03% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
app/app.go	71.24% <100.00%> (+0.07%)	⬆️

... and 319 files with indirect coverage changes

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