Remove dead placeholder / prompt param

[stephencattaneo]

the x-select component code has placeholder yet the the template doesn't take advantage of it.

[Robdel12]

Good catch. This is actually leftover from when we supported a blockless form of the component. If you still would like to do that you can use x-select-blockless.

[stephencattaneo]

FWIW this change fixes it (app/templates/components/x-select.hbs):

{{#if placeholder}}
  {{#x-option}} {{placeholder}} {{/x-option}}
{{/if}}
{{yield}}

I can make a pull request if you want

[cowboyd]

@stephencattaneo

The philosophy of x-select is to be as thin a wrapper around the native select option as possible. That way, it remains simple, flexible, and clear how to work with it given knowledge of the standard select and option tags.

So would definitely accept a PR to remove the placeholder attribute, but not one to use it in the template.