Remove unused weightSelect from initIssuableSidebar

What does this MR do?

Remove unused weightSelect from initIssuableSidebar.

initIssuableSidebar is imported and invoked in the following 3 different places

1. merge_conflicts/merge_conflicts_bundle.js
2. pages/projects/issues/show/index.js
3. pages/projects/merge_requests/show/index.js

We don't use weights in MRs, so point 1 and 3 shouldn't be doing anything related to weightSelect. As for the issues page, we currently show the weight component in Vue, so the initialization of weightSelect in initIssuableSidebar is unnecessary. The current code calls weightSelect on that page and does nothing since it cannot find any selectors matching .js-weight-select.

Are there points in the code the reviewer needs to double check?

None

Why was this MR needed?

~"technical debt"

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/4923