I'd always recommend keeping Maths and Physics as they compliment each other and both are necessary for engineering. In fact I'd say keep all 4 as they do compliment each other so it's less extra work than you'd expect. Maths helps Physics (and Chemistry to a lesser extent), Physics helps Chemistry and Maths, and Chemistry helps Biology.
If you have to drop one then I'd probably drop Biology based on that as it's less useful than you'd expect for a bioeng degree, particularly if you're not at one of the few unis which does tissue engineering.