Remove Form._clear() method

I had thought this method would be useful, because we would first let a
tax-form's logic set up the form with default input values and then call
_clear() and then let the JSON import logic overwrite the defaults with
any values set explicitly in the JSON.  But it turns out that the way
that "json" builds data structures means they have to be created from
scratch, so we cannot pass an existing Form to the JSON logic, so this
method is now obsolete.