[ios_platform_view] Remove platform view if a duplicate is created.

[cyanglaz]

Fixes flutter/flutter#125913

The fix in this PR is to remove the duplicate PlatformView from the view tree.
See description in the above issue for details.

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[chinmaygarde]

Ping @hellohuanlin

[hellohuanlin]

we sync'ed offline about this PR, and will need to think more about whether to patch the engine, or fix the race in the scenario test which is the root cause

[cyanglaz]

Closing this one for now, I will create a PR to fix the scenario test first.