Investigate removal of GlobalLabel

GlobalLabel is used only by global milestone, we could probably remove this model (as it's used only for grouping labels by title), we could remove also dashboard label controller.

The following discussion from !10862 (merged) should be addressed:

• @engwan started a discussion: (+1 comment)

  Is this model still being used? Should we create a follow-up to clean this up?

Also briefly discussed in https://gitlab.slack.com/archives/C72HPNV97/p1555997520092800