Adjusting a triple front derailleur after removing largest chainring I have a touring/commuting bike with full Shimano 105 -- triple in the front.

I just removed the outer (largest) chainring and replaced it with a chain guard. 

It's easy enough to crank on the limit screws to prevent myself from shifting past the middle chainring. However, the chain rubs the front derailleur in my new largest gear (42x11). 

So I'm wondering what the best way to adjust the front derailleur would be. Ideally, I'd like to limit out the lowest shift so that I can take advantage of the trim shift that 105 derailleur and shifter have in the outermost position. Is that possible?

Another option is that I have a 105 double front derailleur hanging around. Perhaps that would be more appropriate.

Side note: I checked this question, but it's replacing the entire crank, while I have simply removed one of the chainrings.
The chain line expected by a double front derailleur is basically what comes with the largest two chain rings on a triple.  Since you are using the smallest two chain rings, the double front derailleur will not shift optimally.

Instead, I'd suggest keeping the triple front derailleur in place.  Using the limit screw -- as you already have -- is the best option.  Given that you are unable to use the trim shift, at the moment, I believe that you may want to back off the limit screw just a bit.  In doing so, you'll allow the front derailleur to move outward a little more, which should allow the trim to work properly.