Remove unused variables with the Babel plugin

[nicolo-ribaudo]

• functions were already removed
• variables can be removed when their initializer does not have side effects
• classes can be removed when they have no static blocks

Fixes #21337

[nicolo-ribaudo]

Oh well, GitHub actions is down.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.42%. Comparing base (03eda70) to head (2691868).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21339      +/-   ##
==========================================
- Coverage   81.42%   81.42%   -0.01%     
==========================================
  Files         257      257              
  Lines       65351    65351              
==========================================
- Hits        53210    53209       -1     
- Misses      12141    12142       +1     

Flag	Coverage Δ
integrationtest	67.00% <ø> (ø)
unittest	56.87% <ø> (-0.01%)	⬇️
unittestcli	56.07% <ø> (-0.03%)	⬇️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/8358dc000813fef/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/6297e9a7697e0d8/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/8358dc000813fef/output.txt

Total script time: 18.24 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/6297e9a7697e0d8/output.txt

Total script time: 25.50 mins

• Regression tests: Passed

[Snuffleupagus]

r=me, thanks.

[Snuffleupagus]

Despite all tests passing, it seems that this broke the GENERIC viewer since opening https://mozilla.github.io/pdf.js/web/viewer.html now fails:

Uncaught (in promise) ReferenceError: eventBus is not defined
    bindEvents app.js:2112
    initialize app.js:269
    run app.js:807
    webViewerLoad viewer.js:374
    <anonymous> viewer.js:385
app.js:2112:5

@nicolo-ribaudo Should we revert this for now, to not block other work, or could this be fixed quickly?