Frontend: Remove -disable-print-package-name-for-non-package-interface.

It is unsound to expose `package` declarations in textual interfaces without a
package identity for them to belong to so we should not offer this flag.

Resolves rdar://139361524.

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -72,10 +72,6 @@ struct ModuleInterfaceOptions {
   /// Print imports that are missing from the source and used in API.
   bool PrintMissingImports = true;
 
-  /// If true, package-name flag is not printed in either public or private
-  /// interface file.
-  bool DisablePackageNameForNonPackageInterface = false;
-
   /// Intentionally print invalid syntax into the file.
   bool DebugPrintInvalidSyntax = false;