Revert "(De-)serialization for foreign error conventions."

This reverts commit face8b23dc1b7132039e8b20b96e9fabe4634649.

Swift SVN r27295

include/swift/Serialization/ModuleFile.h

@@ -660,9 +660,6 @@ public:
   GenericParamList *maybeReadGenericParams(DeclContext *DC,
                                      llvm::BitstreamCursor &Cursor,
                                      GenericParamList *outerParams = nullptr);
-
-  /// Reads a foreign error conformance from \c DeclTypeCursor, if present.
-  Optional<ForeignErrorConvention> maybeReadForeignErrorConvention();
 };
 
 } // end namespace swift