Remove the unused key3 parameter from the various Dict get-methods

[Snuffleupagus]

PR #20542 removed the only spot in the code-base where a Dict get-method was invoked with three keys, hence we can remove a little bit of effectively dead code.

Note that for any "regular" Dict lookup there can be at most two keys, since some PDF properties have shorthand names (e.g. /CS respectively /ColorSpace).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.20%. Comparing base (0f26334) to head (2ae5995).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21815   +/-   ##
=======================================
  Coverage   90.20%   90.20%           
=======================================
  Files         264      264           
  Lines       67339    67335    -4     
=======================================
- Hits        60741    60738    -3     
+ Misses       6598     6597    -1     

Flag	Coverage Δ
browsertest	66.31% <100.00%> (-0.02%)	⬇️
fonttest	8.91% <0.00%> (+<0.01%)	⬆️
integrationtest	69.40% <100.00%> (+<0.01%)	⬆️
unittest	58.36% <100.00%> (-0.01%)	⬇️
unittestcli	56.92% <100.00%> (-0.01%)	⬇️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

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Received

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

Live output at: http://54.241.84.105:8877/8eca4f2358ff526/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

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

Live output at: http://54.193.163.58:8877/7970b48e9ec5c57/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

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Success

Full output at http://54.241.84.105:8877/8eca4f2358ff526/output.txt

Total script time: 18.09 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/7970b48e9ec5c57/output.txt

Total script time: 23.42 mins

• Regression tests: Passed