Remove logic verifying if the step is valid on __unicode__.
Remove logic verifying if the step is valid on __unicode__.

This was removed because all the time the workflow need to
call str, the valid method was called. This is bad because
is_valid is called when we do a retry of another step. And
some is_valid method has slow execution.

globocom/database-as-a-service,globocom/database-as-a-service,globocom/database-as-a-service,globocom/database-as-a-service