Double scroll bar in labels dropdown

Summary

The labels dropdown as of GitLab v10.7 now shows two scrollbars instead of one in v10.6 and below. I haven't tested whether this is true for the milestones dropdowns only for the labels, since this is mostly the only one we're using in our company.

Steps to reproduce

Add multiple labels until a scrollbar is needed (we usually have 5 in ours repositories).

What is the current bug behavior?

Two scrollbars appear.

What is the expected correct behavior?

The labels dropdown should only have one scrollbar.

Relevant logs and/or screenshots

Output of checks

This bug happens on Gitlab.com (see https://gitlab.com/cH40zLord/omnibus-gitlab/issues/new) and on Gitlab EE

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)