Remove package opn

[DonJayamanne]

For #4322

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• [n/a] Has a news entry file (remember to thank yourself!)
• [n/a] Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• [n/a] Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)

[codecov]

Codecov Report

Merging #4538 into master will increase coverage by 21%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #4538     +/-   ##
========================================
+ Coverage      58%     78%    +21%     
========================================
  Files         367     443     +76     
  Lines       15786   20878   +5092     
  Branches     2461    3372    +911     
========================================
+ Hits         9052   16169   +7117     
+ Misses       6142    4702   -1440     
+ Partials      592       7    -585

Flag	Coverage Δ
#Linux	66% <ø> (?)
#Windows	66% <ø> (?)
#macOS	66% <ø> (?)