Locked milestone should not be remove-able for milestone-associated board

• For boards that are associated with a board, the milestone filter is locked into the search bar.
• The milestone filter should not be able to removed.
• With the 9.2 release that includes the x, you can now remove it by clicking x. (https://gitlab.com/gitlab-org/gitlab-ce/issues/30466#note_28308230).
• This should not be the case. The x should not be there in this case.