Remove libgtk install

[baggepinnen]

It started failing

[JuliaControlBot]

Something failed when generating plots. See the log at https://github.com/JuliaControl/ControlExamplePlots.jl/actions/runs/1696614764?check_suite_focus=true for more details.

[codecov]

Codecov Report

Merging #601 (879daf1) into master (32b4e8e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #601   +/-   ##
=======================================
  Coverage   85.70%   85.70%           
=======================================
  Files          31       31           
  Lines        3225     3225           
=======================================
  Hits         2764     2764           
  Misses        461      461           

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