Remove max select for multiple assignees

What does this MR do?

Remove max select for multiple assignees. It was added into EE as a bad CE => EE merge.

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

Shouldn't be

Why was this MR needed?

Allows multiple assignees for issues in EE

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2418 (closed)