Remove unused dead singular assignee code in boards

What does this MR do?

Remove some dead unused code that was still referring to assignee (we have moved to assignees)

Introduced in https://gitlab.com/gitlab-org/gitlab-ce/commit/adc59353fa48c5e433b80efc1698c0b12e121968#852fdbe4145be33eca58d30d8c2f4a6623ef8602_30_30

Only usage removed in https://gitlab.com/gitlab-org/gitlab-ce/commit/de9e559d4495560095adfa03ea6489ea51f6f2f4#e22df3c6b3631af90cde45bbb974ddce72aefab7_21_21

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

Does this MR meet the acceptance criteria?

• Tests
  • 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