Stop exporting addDismissFlashClickListener

What does this MR do and why?

addDismissFlashClickListener was used outside the flash.js module in main.js. It was not working as intended, because the flash elements created by Rails don't have .js-close-icon buttons, but .js-close.

Additional reasons on why addDismissFlashClickListener should be removed in this !108827 (comment 1242289219)

Related to Resize component on alert dismiss (!108811 - merged)

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How to set up and validate locally

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