I don't think that there are LEDs that are in the germicidal UV range. All the germicidal UV lamps I have seen have been mercury vapor, which is basically a flourescent without the phosphor coating on the inside. They also have to be made from quartz, not glass, since ordinary glass will absorve these wavelengths.

I don't know the UV properties of glass used to make lenses, but most likely it won't transmit the short wave UV anyway, whether you make the UV rays with a mercury vapor tube, LED, or some other method.

Added:

Apparently some LEDs are available in the wavelength range that will probably kill fungus, as noted by Caleb in a comment below. However, the problem of getting that UV thru the lens glass to the fungus is still a problem.

Perhaps prolonged exposure to longer wavelengths that the glass can transmit will kill the fungus too, but I don't know that. I suppose there is little harm in leaving the the lens pointed at the sun for a prolonged time, as long as you take care to make sure the rays don't end up focused anywhere they could cause trouble. You'd probably have to use something like a telescope mount to keep the lens pointed at the sun for a few hours.

In any case, killing the fungus already inside the lens at best just won't make it grow bigger. Whatever fungus is already there, with its associated optical degradation, will remain.