Sand or scrape it. Or a strong solvent while wearing nitrile gloves.
Scraping off. Heating with an blow dryer to soften the adhesive for scraping.
Yes, scrape with a big steel blade.
You might try vinegar w boiling hot water. Be careful though, it might damage the grout. Maybe with cloth and by hand.
I assume you want it gone because you'll tile or carpet over it. If it's a large amount over a big area you may have to rent a floor buffer that comes with an industrial scratch type pad to really grind it off there. I think that's what they use before they buff and seal concrete floors.