If you've no desire to reinstall said grommet, how about an X-acto knife to carefully cut the grommet and allow you to peel it off the glass, assuming it hasn't been affixed with something like a cyanoacrylate adhesive? If you suspect it has been glued, then finding the appropriate solvent is your next step.
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There ain't nothin' more powerful than the odor of mendacity -Big Daddy
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