Fix local variables which shadow global variables.

Since the following local variables inside let special form are declared
`nil` with same name as global variables, same name global variables
 is always `nil`.
- `helm-delicious-user`
- `helm-delicious-password`

From the above, if set up `helm-delicious-user' and
`helm-delicious-password' by init file, helm-delicious.el can't work.