plants: Remove unnecessary cmake_minimum_required call
plants: Remove unnecessary cmake_minimum_required call

This function only needs to be called in directories that may be used as
top-level source trees.

billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake