Prune morley-multisig package

Description

Problem: morley repo is quite big, we'd like to split it as much as possible.

Solution: Move morley-multisig package to a dedicated repo https://gitlab.com/morley-framework/morley-multisig/ and prune it from the morley repo.

See morley-multisig!1 (merged).

Related issue(s)

https://issues.serokell.io/issue/TM-481

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.