[Task]: Remove pubsub lite support as it is being deprecated

[shunping]

What needs to happen?

We will need to remove any pubsub lite code (https://github.com/search?q=repo%3Aapache%2Fbeam%20pubsublite&type=code) by March 18, 2026, as pubsub lite is deprecated.

See https://docs.cloud.google.com/pubsub/lite/docs

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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