[in_app_purchase] Fix Memory Leak

[yimao009]

Create a flutter module that depends on the in_app_purchase plugin, and use the pod in my ios native project to use the module. When the flutter engine is created, it is found that it cannot be destroyed. The registry of the in_app_purchase plugin will hold the flutter engine, so it cannot be released.
I don't understand why this in-app purchase plugin calls [registrar textures],

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[cyanglaz]

Thanks for sending out this PR! Do you mind creating an issue explain the issue and details?

XCUITests could be added to test these things too.

[cyanglaz]

Looks like registry is unused in the plugin code and can be completely removed. "messenger" can also be removed with a little tweak in the code.

And it looks like registrar can be weak as the engine should keep a strong reference to the registrar and plugin should out live the engine.

[cyanglaz]

Dup of #5358, since the other one has CLA, im going to close this in favor of the other