FormPhoneInput: Remove LinkedStateMixin

[ockham]

This is obsolete, since FormPhoneInput doesn't use the this.linkState() method provided by the mixin, but instead handles the relavant logic itself in its countryValueLink and phoneValueLink objects -- very much like the textbook example, https://facebook.github.io/react/docs/two-way-binding-helpers.html#reactlink-without-linkedstatemixin

Also, LinkedStateMixin is deprecated as of React 15, so it's a good thing to remove it.

Additionally, this PR changes the initial value of the FormFields example' FormPhoneInput to a valid format (cf p6jOYE-sc-p2).

Code QA:

• Verify that my above claim is true :-) (no reference to this.linkState() in FormPhoneInput)

To test:

• Go to http://calypso.localhost:3000/devdocs/design/form-fields
• Verify that you can still change the phone number in the phone number field below 'Form Phone Input', and change the country code in the dropdown.

[ockham]

/cc @gwwar @aduth for review

[gwwar]

👍 Verified that updating a phone number and phone validation still works. The example also now matches what we see on the phone update form.