[iOS] Remove dead RasterThreadMerger plumbing from platform views

[cbracken]

IOSExternalViewEmbedder::SupportsDynamicThreadMerging() returns false, so the raster thread merger is never engaged on iOS.

This deletes two no-op overrides in FlutterPlatformViewsController:

• postPrerollActionWithThreadMerger: ignores its merger argument and unconditionally returns PostPrerollResult::kSuccess, which is exactly the base-class default of ExternalViewEmbedder::PostPrerollAction.
• `endFrameWithResubmit:threadMerger: has an empty body, matching the base-class default of ExternalViewEmbedder::EndFrame.

Also deletes the IOSExternalViewEmbedder overrides that existed only to forward to them. We now fall back to the base-class defaults which were identical. BeginFrame is pure virtual, so we still need its empty override. This removes fml::RefPtrfml::RasterThreadMerger and flutter::PostPrerollResult from the ObjC embedding surface.

This is pure no-op cleanup stemming from the fact that we now run only in thread-merged mode on iOS, and cleanup that makes an eventual migration to the embedder API easier.

No test changes since there are no semantic changes; covered by existing tests.

Issue: #112232

Pre-launch Checklist

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• I listed at least one issue that this PR fixes in the description above.
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• 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.
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[gemini-code-assist]

Code Review

This pull request removes the PostPrerollAction and EndFrame methods from IOSExternalViewEmbedder, along with the corresponding postPrerollActionWithThreadMerger: and endFrameWithResubmit:threadMerger: methods from FlutterPlatformViewsController. I have no feedback to provide as there are no review comments.

[stuartmorgan-g]

test-exempt: removing code

[hellohuanlin]

LGTM!