[FLINK-4866] [streaming] Make Trigger.clear() Abstract to Enforce Implementation

[wuchong]

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This PR makes Trigger.clear() method to be abstract, so that implementors of custom triggers will not forget to clean up their state/timers properly.

[aljoscha]

Thanks for your work. 👍 I'll be merging this tomorrow.

[aljoscha]

I merged this. Could you please close the issue?

[wuchong]

Sure. I will close this.