Fix <rdar://problem/20193929> Can't "return nil" in a convenience initializer

This falls out of the rework I did of 'self' in initializers.  We now correctly
dealloc_ref the allocated object on the failure path of a convenience init.



Swift SVN r27752

include/swift/AST/DiagnosticsSIL.def

@@ -167,9 +167,6 @@ ERROR(assignment_to_immutable_value,sil_analysis,none,
       "immutable value '%0' may not be assigned to",
       (StringRef))
 
-ERROR(self_init_must_be_called_before_failure,sil_analysis,none,
-      "failable convenience initializer must delegate to self.init() before "
-      "returning nil", ())
 ERROR(object_not_fully_initialized_before_failure,sil_analysis,none,
       "all stored properties of a class instance must be initialized before "
       "returning nil from an initializer", ())