Drop support for mm.LinkTo component, use LinkTo with on click modifier instead

[nickschot]

Previously we supported:

<mmw.MobileMenu @isOpen={{this.isOpen}} @onToggle={{this.onToggle}} as |mm|>
  <mm.LinkTo @route="index">Home</mm.LinkTo>
</mmw.MobileMenu>

Which relied on https://github.com/emberjs/ember-legacy-built-in-components. This PR removes the custom (extended) LinkTo component entirely.

We now recommend to use the default LinkTo with an {{on "click"}} modifier and call the close action directly:

<mmw.MobileMenu @isOpen={{this.isOpen}} @onToggle={{this.onToggle}} as |mm|>
  <LinkTo @route="index" {{on "click" mm.actions.close}}>Home</LinkTo>
</mmw.MobileMenu>