refactor: remove unnecessary config.Seal() (backport #3786)

[mergify]

• Add unit tests to cosmos SDK config set up
• Remove an unnecessary config.Seal invocation that has been around for a long time but isn't necessary because the init() command always sets and seals the config.

  ---

  This is an automatic backport of pull request refactor: remove unnecessary config.Seal() #3786 done by Mergify.

[mergify]

Cherry-pick of 9c9e77a has failed:

On branch mergify/bp/v2.x/pr-3786
Your branch is up to date with 'origin/v2.x'.

You are currently cherry-picking commit 9c9e77a2.
  (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:
	deleted:    app/config.go
	new file:   app/sdk_config.go
	new file:   app/sdk_config_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cmd/celestia-appd/cmd/root.go

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