iOS: Remove the non-merged thread topology

[cbracken]

iOS runs with the platform and UI threads merged, and it's not possible to create an engine with unmerged platform/ui threads, but FlutterEngine still included branches to create the unmerged threads when Settings::merged_platform_ui_thread wasn't enabled.

The option to opt-out of merged threads on iOS was removed in #174408 in Aug 2025, and it had been the default configuration for over a year at that point.

Both existing routes to a non-merged thread configuration already failed at startup:

• Setting FLTEnableMergedPlatformUIThread=false in Info.plist causes an FML_CHECK in FlutterDartProject.mm to blow up.
• Setting --no-enable-merged-platform-ui-thread or --merged-platform-ui-thread=disabled results in them being parsed into Settings::merged_platform_ui_thread in SettingsFromCommandLine, and triggers the same FML_CHECK.

The only route that didn't trigger an abort() on startup was -[FlutterDartProject initWithSettings:], which is declared in FlutterDartProject_Internal.h and isn't part of the framework's public API.

This also removes the last place the iOS embedder checked Settings::enable_impeller. Now it's simply assumed.

This is part of cleanup work intended to simplify the iOS embedder prior to an eventual migration to the embedder API.

Issue: #112232

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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Code Review

This pull request removes the option to unmerge the platform and UI threads on iOS, ensuring they are always merged. It removes the _uiTaskRunnerWrapper member variable, simplifies MakeThreadHost by removing the settings parameter and the creation of a dedicated UI thread, and updates createShell: to use the platform thread's task runner for the UI task runner. Additionally, the test testCanUnMergePlatformAndUIThread has been removed. There are no review comments, and I have no feedback to provide.