refactor(testutil)!: remove cmdtest (backport #22826)

[mergify]

Description

Reduces amount of testing framework. Similar to #22392.
This isn't used by anyone and the alternative is a two liner or using systemtests / intergration tests framework.

(reverts #15251)

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Summary by CodeRabbit

• New Features

  • Simulated nested messages in the base application.
  • Introduced a new Linux-only backend for crypto/keyring support.
  • Added support for importing hex keys via standard input.
  • Enhanced version command to return v2 server information.

• Improvements

  • Upgraded RocksDB libraries to version 9.
  • Simplified testing frameworks by removing unnecessary components.

• Bug Fixes

  • Resolved issues in simulation tests and command-line interface queries.

• Documentation

  • Updated CHANGELOG.md to reflect new features, improvements, and bug fixes.

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This is an automatic backport of pull request #22826 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 71d2571 has failed:

On branch mergify/bp/release/v0.52.x/pr-22826
Your branch is up to date with 'origin/release/v0.52.x'.

You are currently cherry-picking commit 71d2571e2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   client/keys/add.go
	modified:   client/keys/export.go
	modified:   client/keys/mnemonic.go
	modified:   client/keys/show.go
	modified:   client/keys/utils.go
	modified:   tests/go.mod
	deleted:    tests/integration/genutil/export_test.go
	deleted:    testutil/cmdtest/system.go
	modified:   version/command.go
	modified:   version/version_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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