Remove proposal, broken out into apple#683

Author: czechboy0

Sources/swift-openapi-generator/Documentation.docc/Proposals/Proposals.md

@@ -54,4 +54,3 @@ If you have any questions, tag [Honza Dvorsky](https://github.com/czechboy0) or
 - <doc:SOAR-0010>
 - <doc:SOAR-0011>
 - <doc:SOAR-0012>
-- <doc:SOAR-0013>