[FLINK-23308] Optimize checks if SourceContext is closed

[dawidwys]

• Remove unnecessary calls to checkNotClosed
• Make the checkNotClosed private to enable inlining

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[pnowojski]

LGTM % one question, and commit message/PR description should explain more what is this about (trying to optimise potential regression and you are not sure whether this is fixing something, but you want to try it out over a couple of days on the continuous benchmarking machines).

[dawidwys]

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CI report:

• 00aeb24 Azure: CANCELED Azure: SUCCESS

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