[google_maps_flutter_web] Stop listening to map events when disposed

[JamesMcIntosh]

flutter/flutter#168782
flutter/flutter#159388
flutter/flutter#75818

Pre-Review Checklist

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• [/] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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