v3: remove all deprecated methods re obsolete push API

[bigbes]

The legacy push notification mechanism—based on box.session.push has been fully replaced by the Watcher interface. All code supporting this obsolete push API is now deprecated and should be **completely removed.

Affected Files

• go-tarantool/future.go
• go-tarantool/response_it.go

What to Remove

Identify all types, methods, and interfaces in these files that are:

• Marked with // Deprecated:
• Part of the old push/event-listening flow (e.g., Future.SetPushCallback, ResponseIterator, OnPush, etc.)
• No longer used by any current public API or internal logic

Clean up dependencies and update tests.

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Duplicates https://github.com/tarantool/go-tarantool/issues/324