Revert "[web] Better way to detect CanvasKit variant"

[gaaclarke]

Reverts #40154

Link to failure:
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20Web%20Engine/15924/overview

failure text:

05:04 �[32m+312�[0m�[33m ~14�[0m�[31m -4�[0m: test/canvaskit/image_golden_test.dart: CanvasKit Images (webcodecs) the same image can be rendered on difference surfaces�[0m                                                           
05:04 �[32m+312�[0m�[33m ~14�[0m�[31m -4�[0m: �[1m�[90mloading test/ui/path_test.dart�[0m�[0m                                                                                                                                                      
goldctl imgtest add stdout: Given image with hash efdfae048b059fa1fdd0cd04a046b5a5 for test canvaskit_read_back_decoded_image_webcodecs
Expectation for test: 232f176cb03fce28273397dea4d5d125 (positive)
Expectation for test: efdfae048b059fa1fdd0cd04a046b5a5 (negative)
Untriaged or negative image: https://flutter-engine-gold.skia.org/detail?test=canvaskit_read_back_decoded_image_webcodecs&digest=efdfae048b059fa1fdd0cd04a046b5a5


goldctl imgtest add stderr: Test: canvaskit_read_back_decoded_image_webcodecs FAIL

[gaaclarke]

Landing on red to open the tree.