feat!: use built-in cobra completion command

[aaronc]

Description

I noticed that in simd we're using tmcli.NewCompletionCmd instead of cobra's built-in completion command. Maybe at some point cobra didn't have this functionality, but now the built-in functionality is better and also supports fish and powershell. So we should encourage apps to just use default cobra completion.

Not sure if any CHANGELOG entry is needed because this is just simapp.

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Author Checklist

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• provided a link to the relevant issue or specification
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
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• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov]

Codecov Report

Merging #13548 (3114340) into main (fa2cb40) will decrease coverage by 0.11%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13548      +/-   ##
==========================================
- Coverage   53.93%   53.82%   -0.12%     
==========================================
  Files         643      653      +10     
  Lines       55464    56825    +1361     
==========================================
+ Hits        29916    30586     +670     
- Misses      23148    23795     +647     
- Partials     2400     2444      +44     

Impacted Files	Coverage Δ
simapp/simd/cmd/root.go	70.23% <ø> (ø)
crypto/keys/internal/ecdsa/privkey.go	81.13% <0.00%> (-1.89%)	⬇️
x/staking/simulation/operations.go	74.54% <0.00%> (-1.38%)	⬇️
simapp/state.go	0.00% <0.00%> (ø)
simapp/app.go	77.84% <0.00%> (ø)
simapp/genesis_account.go	100.00% <0.00%> (ø)
simapp/simd/cmd/genaccounts.go	89.09% <0.00%> (ø)
simapp/genesis.go	100.00% <0.00%> (ø)
simapp/export.go	11.94% <0.00%> (ø)
simapp/test_helpers.go	30.25% <0.00%> (ø)
... and 3 more