[FLINK-20807][build] Remove/narrow various checkstyle suppressions

[zentol]

A whole bunch of checkstyle suppressions are no longer necessary, or could be narrowed down to single files.
So I did just that.

This was an interesting exercise in which parts are applied by spotless, and which still require checkstyle to be active.

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[tillrohrmann]

Eyeballing these changes and looking at AZP passing, I think it is good to be merged.