refactor: simplify sign mode (2/2) (backport #23476)

[mergify]

Description

we actually do not need to have those converting helpers at all.
Especially internally when someone can extend the sign mode

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Summary by CodeRabbit

Release Notes

• Improvements

  • Simplified sign mode handling across the SDK
  • Streamlined API interactions related to signature modes

• Removed Functionality

  • Eliminated APISignModeToInternal and InternalSignModeToAPI conversion methods
  • Removed unnecessary intermediate sign mode conversion steps

• Technical Updates

  • Updated import and usage of sign mode types from new API package
  • Direct type casting replaced complex conversion logic

These changes simplify the SDK's signing process, making it more straightforward for developers to work with transaction signatures.

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This is an automatic backport of pull request #23476 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 39b0244 has failed:

On branch mergify/bp/release/v0.52.x/pr-23476
Your branch is up to date with 'origin/release/v0.52.x'.

You are currently cherry-picking commit 39b0244ee.
  (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/tx/aux_builder.go
	modified:   client/tx/aux_builder_test.go
	modified:   types/tx/direct_aux.go
	modified:   types/tx/direct_aux_test.go
	modified:   types/tx/signing/signature.go
	deleted:    types/tx/signing/types.go
	modified:   x/auth/tx/sigs.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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