remove alias from RecurrentOp to make user interface more concise

[Superjomn]

When RecurrentOp is implemented, variables in child-scope might conflict with global scopes, so alias of variables is necessary.

Currently, the local scope can have variables that have the same names with some variables in global scope.

When design user interface, I found that the alias is necessary to define a step-net, so, it is better to remove alias and make RNNOp's user interface more concise.