Addon-backgrounds: remove redundant scrollbars

[Hypnosphi]

Fixes the first part of #2655

How to test

Open addon-backgrounds example

Note for mac users: to see the difference, you may need to go to Settings -> General and select "Always" as "Show scroll bars" option

[codecov]

Codecov Report

Merging #2744 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2744   +/-   ##
=======================================
  Coverage   33.02%   33.02%           
=======================================
  Files         419      419           
  Lines        9158     9158           
  Branches      971      971           
=======================================
  Hits         3024     3024           
  Misses       5485     5485           
  Partials      649      649

Impacted Files	Coverage Δ
addons/background/src/index.js	86.36% <ø> (ø)	⬆️
app/react/src/server/utils.js	0% <0%> (-53.58%)	⬇️
app/vue/src/server/utils.js	53.57% <0%> (+53.57%)	⬆️

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[ndelangen]

Merge when ready