SE-0157: Enable recursive protocol constraints by default.

We don't know if they work in general, but specific cases of recursive
protocol constraints are working, and enabling them doesn't break
anything.

include/swift/Basic/LangOptions.h

@@ -210,9 +210,6 @@ namespace swift {
     /// Should we use \c ASTScope-based resolution for unqualified name lookup?
     bool EnableASTScopeLookup = false;
 
-    /// Enable SE-0157: Recursive Protocol Constraints.
-    bool EnableRecursiveConstraints = false;
-
     /// Whether to use the import as member inference system
     ///
     /// When importing a global, try to infer whether we can import it as a