removed PrepareProposal from Application

[pompon0]

PrepareProposal in sei-chain Application is a noop, so I've removed it entirely to simplify the logic.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 24, 2026, 6:46 PM

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.65%. Comparing base (1141939) to head (13b28d3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/benchmark/generator.go	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3096      +/-   ##
==========================================
+ Coverage   58.62%   58.65%   +0.02%     
==========================================
  Files        2099     2099              
  Lines      173751   173511     -240     
==========================================
- Hits       101867   101776      -91     
+ Misses      62816    62690     -126     
+ Partials     9068     9045      -23     

Flag	Coverage Δ
sei-chain-pr	65.74% <89.47%> (?)
sei-db	70.41% <ø> (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	67.60% <ø> (+0.16%)	⬆️
app/benchmark.go	48.93% <ø> (+4.10%)	⬆️
app/benchmark/benchmark.go	60.00% <100.00%> (ø)
sei-cosmos/baseapp/abci.go	61.42% <ø> (+0.58%)	⬆️
sei-cosmos/baseapp/baseapp.go	74.14% <ø> (-0.47%)	⬇️
sei-cosmos/baseapp/options.go	59.49% <ø> (+0.23%)	⬆️
sei-tendermint/abci/types/application.go	48.38% <ø> (+13.50%)	⬆️
sei-tendermint/abci/types/mocks/application.go	13.12% <ø> (-2.36%)	⬇️
sei-tendermint/abci/types/plain_types.go	5.82% <ø> (-0.33%)	⬇️
sei-tendermint/internal/proxy/client.go	65.11% <ø> (-2.28%)	⬇️
... and 6 more

... and 1 file with indirect coverage changes

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