Fix Assignee dropdown showing assignee(s) twice

What does this MR do?

• Fixes: double divider and assignee(s) headers on the assignee the issues/merge requests assignees list when visiting for the 2nd time(click edit once, exit the dropdown, then click edit again)

Test locally on GDK

• Go to any issue or merge request
• Click edit on assignees - observe.
• Close assignee dropdown.
• Click edit on assignee again and notice no duplication of divider/header.

Screenshots (strongly suggested)

before	after
Screen_Recording_2021-03-31_at_2.54.32_PM	Screen_Recording_2021-03-31_at_2.47.56_PM

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #325991