Remove close icon from projects dropdown in issue boards

What does this MR do?

Remove close icon from projects dropdown in issue boards which doesn't comply with dropdown design specs.

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23546#note_122169429

part of https://gitlab.com/gitlab-org/gitlab-ee/issues/7715

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer