[connectivity_platform_interface] Remove Platform asserts.

[ditman]

Description

Even though the code path never hits web targets, having dart:io imports in the MethodChannel implementation in the platform_interface package breaks the pana analysis of federated implementations for platforms where those imports are forbidden (like web), with something similar to this:

$~/connectivity/experimental_connectivity_web does not support flutter with runtime: flutter-web because of import violation at: 

package:experimental_connectivity_web/experimental_connectivity_web.dart,
package:experimental_connectivity_web/src/network_information_api_connectivity_plugin.dart,
package:experimental_connectivity_web/src/utils/connectivity_result.dart,
package:connectivity_platform_interface/connectivity_platform_interface.dart,
package:connectivity_platform_interface/src/method_channel_connectivity.dart, 
dart:io

This change removes the Platform check assert (if deemed critical, it should be added to the connectivity plugin, or maybe hidden behind a conditional import in this package).

All current tests, both in connectivity_platform_interface and connectivity pass.

Related Issues

n/a

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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

• No, this is not a breaking change.

[cyanglaz]

LGTM