Discourage use of `optional_aes` field

The Geom$optional_aes and Stat$optional_aes fields seem redundant to me.
Instead of having these fields, we can declare default_aes = aes(optional_aesthetic = NULL).
While I would like to see these fields go in their entirety, that seems like backward incompatible, so instead I propose to discourage these fields. We can (i) remove any references to that field in the extension vignette (ii) ensure we adopt the NULL-aesthetic approach within ggplot2 and (iii) check if the ggplot book would need to be adapted.
If we're in a violent mood we can start throwing deprecation warnings about that field, but it seems sensible to discourage first and reconsider the next release cycle.

Search the extension vignette and ggplot book for references to Geom$optional_aes and Stat$optional_aes, then inspect ggplot2's default_aes declarations for the NULL-aesthetic approach. Done means the documentation no longer promotes optional_aes and ggplot2's own declarations consistently use the proposed approach; deprecation warnings are explicitly optional.