Repeate after me: This is not JS

This fixes the case when the user wants to add a label. The user has to use the dropdown’s filter input to look for a label and click it in order to see the bug.

Step to reproduce

- Select at least two issues, one label should be present in all issues, other label should be present in at least one.

- On the label dropdown: Deselect label that is present in all issues, look for another issue using the filter input and click it.

- Click on `Update issues`

Before: Unmarked label were kept on selected issues.
Now: Unmarked label is removed from selected issues

This reverts commit 13e37a3e.

Fixes the case when we want to assign a label to multiple issues and one of the issues has already the label we want to apply.