Remove plugin support from the future work section

Author: ncooke3

proposals/0403-swiftpm-mixed-language-targets.md

@@ -506,7 +506,6 @@ listed in the Future Directions section as an area of future work.
 
 ## Future Directions
 
-- Enable package manager plugin tools to process mixed language targets.
 - Enable package authors to expose non-public headers to their mixed
   target's Swift implemention.
 - Investigate uses cases for extending mixed language target support to