You should be able to get pretty close with a cutoff wheel, then use a heavier grinding wheel to get even further, then a flap wheel to finish, like @dx250 said.
You can cut and grind w your angle grinder and remove the lip...
You can cut around the keg and remove the top chine for a complete open top, then bolt some handles on and make it more like a kettle...
But in the end you will still have a keg re purposed into a kettle that is too heavy w a bottom chine that makes a marginal brew kettle.
My suggestion is to sell as is for $50 bucks and buy a shiny new concord 15 gal kettle w lid on eBay for 90 bucks....
Much easier less sparks and overall a much better vessel for brewing beer for net 40 bucks...no need to buy cut off wheels flap disks and no sparks in the face lol
Jmo...I don’t feel kegs make good kettles for many reasons.
Sorry jmo
I want to remove it so I can better fit a biab basket or if I decide to stick with bags, the bags won’t tear on the lip that remains on the keggle.
However after looking at prices for a custom made keggle sized biab ss basket, id probably save money by getting an off the shelf kettle/biab basket combo.
Are you planning to convert it to RIMS or Herms? If not, you'd be better off using a cooler to mash in IMO.
I have a keggle but only use it as a HLT and occasionally a BK. I've never mashed in it. Not sure how well it would hold temps and my cooler does a great job of that.
I replaced my keggle with a 20gal tri-clad-bottom kettle and my experience is that the kettle comes to a boil much faster than the same volume in the keggle.
Not familiar with an “off the shelf, kettle / BIAB basket combo”
If you finish smooth the cut edge of your keggle, you can BIAB with it as is. I make a keggle bag that is tapered such that the bottom of the bag is much narrower than the top, helping to remove the grain bag from the smaller opening w/ typical keggles.