Remove the now-unused flag -enable-experimental-structural-opaque-types

Author: DougGregor

include/swift/Option/FrontendOptions.td

@@ -486,10 +486,6 @@ def enable_experimental_named_opaque_types :
   Flag<["-"], "enable-experimental-named-opaque-types">,
   HelpText<"Enable experimental support for named opaque result types">;
 
-def enable_experimental_structural_opaque_types :
-  Flag<["-"], "enable-experimental-structural-opaque-types">,
-  HelpText<"Enable experimental support for structural opaque result types">;
-
 def enable_explicit_existential_types :
   Flag<["-"], "enable-explicit-existential-types">,
   HelpText<"Enable experimental support for explicit existential types">;