Remove the unused /Decode handling from the JpegStream/JpegImage classes

[Snuffleupagus]

According to the coverage data this code is now unused, see:

• https://app.codecov.io/gh/mozilla/pdf.js/commit/9aea8e2dfb142ad7c4e3a2595e8052e5dc8ce160/blob/src/core/jpeg_stream.js?dropdown=coverage#L74
• https://app.codecov.io/gh/mozilla/pdf.js/commit/9aea8e2dfb142ad7c4e3a2595e8052e5dc8ce160/blob/src/core/jpeg_stream.js?dropdown=coverage#L149
• https://app.codecov.io/gh/mozilla/pdf.js/commit/9aea8e2dfb142ad7c4e3a2595e8052e5dc8ce160/blob/src/core/jpg.js?dropdown=coverage#L1281

I've not attempted to find out exactly when it was made obsolete, however it most likely happened with the introduction of ImageDecoder.

Note that any /Decode entry is already parsed in the PDFImage constructor, see this code, and that the PDFImage.prototype.getImageBytes method is only ever invoked with the forceRGBA/forceRGB parameters set from this code respectively this code.
Note that both of those occurrences are placed within the following block:

pdf.js/src/core/image.js

Lines 834 to 839 in 9aea8e2

	if (
	this.image instanceof JpegStream &&
	!this.smask &&
	!this.mask &&
	!this.needsDecode
	) {

Hence, outside of the IMAGE_DECODERS build, there's no longer any reason for the JpegStream and JpegImage classes to implement their own /Decode handling.
Besides removing what's now dead code, it also improves consistency in the code-base given that none of the other image-decoders do this.

Finally, while slightly unrelated, move SMask/Mask checks in the PDFImage.prototype.createImageData method to occur earlier instead of unnecessarily repeating them later.

---

Slightly smaller diff with https://github.com/mozilla/pdf.js/pull/21809/changes?w=1

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.44444% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (9aea8e2) to head (b1511e5).

Files with missing lines	Patch %	Lines
src/core/jpg.js	33.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21809      +/-   ##
==========================================
- Coverage   90.19%   90.19%   -0.01%     
==========================================
  Files         264      264              
  Lines       67350    67339      -11     
==========================================
- Hits        60749    60734      -15     
- Misses       6601     6605       +4     

Flag	Coverage Δ
browsertest	66.32% <44.44%> (+0.01%)	⬆️
fonttest	8.91% <0.00%> (+<0.01%)	⬆️
integrationtest	69.39% <44.44%> (-0.01%)	⬇️
unittest	58.35% <44.44%> (-0.01%)	⬇️
unittestcli	56.94% <44.44%> (+<0.01%)	⬆️

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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/d2f75828016aa6c/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

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

Live output at: http://54.193.163.58:8877/37edbe470854a82/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/d2f75828016aa6c/output.txt

Total script time: 18.10 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/37edbe470854a82/output.txt

Total script time: 23.13 mins

• Regression tests: Passed

[calixteman]

LGTM. Thank you.