Remove Ubuntu 18 from appveyor.yml.

[kayoub5]

Spin off #495

Ubuntu 18.04 already reached end of life (only paid Extended Security Maintenance is available), so it would make sense to just remove Ubuntu from AppVeyor

Reference: https://ubuntu.com/blog/ubuntu-18-04-eol-for-devices

Resolves #396

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4dfdd42) 82.89% compared to head (e291416) 82.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #497   +/-   ##
=======================================
  Coverage   82.89%   82.89%           
=======================================
  Files          51       51           
  Lines        2823     2823           
  Branches      312      312           
=======================================
  Hits         2340     2340           
  Misses        361      361           
  Partials      122      122           

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