[CQ] remove dead GRADLE_SYNC_TOPIC subscriptions

[pq]

Back In 2019, we added some logic to reflectively access a not-yet public "GRADLE_SYNC_TOPIC" on GradleSyncState subscribing a listener that called GradleUtils.checkDartSupport() on all sync events.

Sometime since then, the field has gone away entirely and we are not getting these notifications. What makes this interesting is how our use of reflection masked the failure.

Luckily, an inspection flagged the unsafe check, spamming our logs at runtime:

My first attempt was to fix the cast but I'm glad I embraced the yak shave. "Safe" reflective access in this case would still have obscured the unnecessariness of this code.

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