Sun_flare -> Lens_flare

BUR #7397 has been rejected.

create alias sun_flare -> lens_flare

seems to be used when lens flare is not known. The effect is the same.

-1. Could be confused for solar flares.

There's one solar flare post on danbooru (and one mistagged lens flare) so I'm not convinced that's a valid concern.

I manually emptied the tag, fixing a few mistags.

The bulk update request #7397 (forum #197583) has been rejected by @nonamethanks.