Remove deprecated --optipng from glue options
Remove deprecated --optipng from glue options

Optipng is now deprecated in glue, so including that option in
Cactus doesn't really make sense.

https://github.com/jorgebastida/glue/blob/master/docs/changelog.rst#09

Bluetide/Cactus,Knownly/Cactus,chaudum/Cactus,fjxhkj/Cactus,danielmorosan/Cactus,danielmorosan/Cactus,juvham/Cactus,Knownly/Cactus,koobs/Cactus,danielmorosan/Cactus,page-io/Cactus,PegasusWang/Cactus,PegasusWang/Cactus,Bluetide/Cactus,andyzsf/Cactus-,andyzsf/Cactus-,chaudum/Cactus,fjxhkj/Cactus,Knownly/Cactus,ibarria0/Cactus,dreadatour/Cactus,koobs/Cactus,eudicots/Cactus,fjxhkj/Cactus,dreadatour/Cactus,juvham/Cactus,PegasusWang/Cactus,koenbok/Cactus,dreadatour/Cactus,koenbok/Cactus,chaudum/Cactus,ibarria0/Cactus,eudicots/Cactus,andyzsf/Cactus-,koenbok/Cactus,koobs/Cactus,eudicots/Cactus,page-io/Cactus,ibarria0/Cactus,page-io/Cactus,Bluetide/Cactus,juvham/Cactus