Reverts "[engine] always force platform channel responses to schedule a task. (#54975)"

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Reverts: #54975

Initiated by: jtmcdole

Reason for reverting: I believe this change caused flutter/flutter to break in an engine roll. It was one of two changes - the other being a webui change.

Tests broken:

module_test_ios: testDualCold

router test:

[2024-09-05 17:43:20.837343] [STDOUT] stderr: [ +135 ms] VMServiceFlutterDriver: Connected to Flutter application.
[2024-09-05 17:43:20.841927] [STDOUT] stdout: [   +4 ms] 00:00 �[32m+0�[0m: 
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Original PR Author: jonahwilliams
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Reviewed By: {jason-simmons, johnmccutchan}
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This change reverts the following previous change:
If we use runNowOrPostTask on platform channel responses, then we may not wake up the dart message loop. If nothing else wakes it up, then the embedder may hang on platform channel responses.

This fixes several google3 integration tests when run in merged thread mode.

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