Selectize backspace issue

Now deletion is disabled for single select input created from `selectInput(..., selectize = TRUE)`.

```s
runApp(list(
  ui = fluidPage(
    selectInput('foo1', 'No deletion, no searching', choices = state.name),
    selectInput('foo2', 'Multiple choices (allows deletion)', choices = state.name, multiple = TRUE),
    selectInput('foo3', 'Original select input', choices = state.name, selectize = FALSE),
    selectInput('foo4', 'Original multiple select', choices = state.name, selectize = FALSE, multiple = TRUE)
  ),
  server = function(input, output) {
    observe({
      print(input$foo1)
      print(input$foo2)
      print(input$foo3)
      print(input$foo4)
    })
  }
))
```

#404