You might be alright doing it to a 5g but generally the frame is a structural part of the tank and removing it could lead to failure later.
But, if you wanna give it a go anyway, get a razor blade and slide it around under the frame inside and out. Also if you have a dremel tool or something similar and don't mind destroying the frame you can make some cuts in the corners to make it easier. This work out pretty well for me when I had to remove the frame to replace it on a tank.
As mentioned they are there for added strength but that mainly applies to bigger tanks. Silicone is more than sufficient to hold these small tanks together the rim servers more to cover the sharp unfinished edges and to complete the look of the tank on these small tanks. People have been derimming smaller tanks ever since the nano reef tank craze and high end planted tanks started years ago. Everything up to 20 longs get done frequently with no issues from what I have read in the past 6 years or so on various forums. Some go bigger but then they always have some deflection numbers that make most uncomfortable. I think most end up in the long run just buying a rimless tank if they like the look.
A sharp razor blade run under the lip to cut the silicon in places will be needed, some use a dremel also to cut the plastic but you have be careful and avoid nicking the glass. The edges will be SHARP is the main thing you need to be aware of. A good pair of leather work gloves should be used during the process.