chore(firebase_core): remove dependency on 'universal_platform'

[mehmetf]

Description

These plugins should be very selective of which packages they depend on since they are core functionality to many Flutter apps. Let's avoid creating dependencies outside of the core Flutter/Dart packages.

Checklist

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• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• No, this is not a breaking change.

[Salakar]

LGTM - this makes, I wasn't aware kIsWeb existed in flutter/foundation.dart - TIL. We'll make sure other plugins don't do this as well. Thank you for the PR