Remove GetSigners from sdk.Msg implementations

[ThanhNhann]

Description

partially addresses: #4687

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md).
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards and Go style guide.
• Wrote unit and integration tests.
• Updated relevant documentation (docs/) or specification (x/<module>/spec/).
• Added relevant godoc comments.
• Provide a commit message to be used for the changelog entry in the PR description for review.
• Re-reviewed Files changed in the Github PR explorer.
• Review Codecov Report in the comment section below once CI passes.

[DimitrisJim]

no worries w.r.t pull request labeler, you can run make lint/make lint-fix for linter complains.

[DimitrisJim]

note, also grepped for it and found GetSigners for a mock message, see: ./modules/apps/27-interchain-accounts/types/codec_test.go:40

I'd assume that can also probably be safely dropped.

[damiannolan]

Awesome! I just commented on some more tests which could be refactored to remove the suite receiver and use moduletestutil.MakeTestEncodingConfig

Happy to merge after those are addressed 🚀

[ThanhNhann]

Updated, thank @damiannolan for your suggestions ❤️

[damiannolan]

ACK finishing touches, thank you @ThanhNhann! 🚀

[DimitrisJim]

lgtm, left one minor nit while going through changes