Remove overridden/duplicated [$getChildren] methods

[Snuffleupagus]

Note how the XFAObject class has two separate [$getChildren] methods, which means that the later one will override the former. Looking at the coverage data confirms that the former method is indeed unused.
Note that normally ESLint will catch that sort of thing, but I guess that the name being determined from a Symbol prevents that (since ESLint is only a static code analyser).

Furthermore the XmlObject class extends XFAObject, however it still defines its own [$getChildren] method that is identical to the one in the base class.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.08%. Comparing base (c2bbde1) to head (a611b78).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21736      +/-   ##
==========================================
- Coverage   90.09%   90.08%   -0.02%     
==========================================
  Files         264      264              
  Lines       67194    67231      +37     
==========================================
+ Hits        60538    60563      +25     
- Misses       6656     6668      +12     

Flag	Coverage Δ
browsertest	66.34% <ø> (-0.05%)	⬇️
fonttest	8.99% <ø> (ø)
integrationtest	69.28% <ø> (+<0.01%)	⬆️
unittest	58.23% <ø> (-0.04%)	⬇️
unittestcli	56.60% <ø> (-0.04%)	⬇️

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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/a776a2c5704fad5/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

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

Live output at: http://54.193.163.58:8877/5c7efcb12ba07b5/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/a776a2c5704fad5/output.txt

Total script time: 16.51 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/5c7efcb12ba07b5/output.txt

Total script time: 23.48 mins

• Regression tests: Passed

[Snuffleupagus]

Rebasing to master, no code changes made.