Can I replace the front front derailleur with chain guide? My front derailleur (Shimano LX T670 / 34.9mm) has been stuck on the highest gear for a couple of weeks now which has got me thinking about how little I use it. To save weight could I replace my front derailleur with  some sort of static chain guard/stay to keep my chain on the same chain ring? Does such a thing exist for three speed chainsets (Shimano T780 48Ax36x26 with chainguard)?

I am not ready to completely convert my bike to a single speed just yet as I imagine I could install the front derailleur back on if I decide to go on a more adventurous journey compared to the daily commute where its not needed.

Update

While the FD is only 155 grams I do enjoy finding ways to save weight here and there, before you know it it's half a KG lighter. The problem with leaving it on is that the spring has rusted into place almost at mid shift, this causes the chain to rub the side of the chain guide which produces an annoying sound and causes it to occasionally jump to the middle chainset. 

For now I have simply removed it but in the past on a different bike I suffered from chain jumping which is what I am trying to avoid by using a chain guide. 

Thanks
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.