Can I replace the front front derailleur with chain guide?
The simple test is just to remove the derailleur. Some of them have a screw holding the bottom of the cage together, in which case it's easy - just undo the clamp holding the derailleur to the seat tube, the cable clamp, then once it's loose undo that screw and wiggle it off the chain. 

Without that cage screw you'll have to break the chain which requires either a quick link that can be undo by hand, or a special tool. Putting it back on afterwards is slightly more complex but quite doable for most people. If that works, you can then remove the cable and outer, and possibly the shifter (these days likely to be built into the brake lever so not removable). 

But as Batman pointed out, the front derailleur is not the heavy part of a multi-speed setup. Most of the weight loss from a single-speed conversion is in losing the extra chain, cassette and rear derailleur, and the maintenance benefits also come mostly from not having to adjust the rear derailleur. So the total benefit is likely to be low. You might also lose a little weight by removing one of both of the extra chainrings, but again not every much.