resource/cloudflare_zone: no need to set rate plans for new free plans

[jacobbednarz]

By default, zones are created with the free rate plan so we don't need
to explicitly make the call a second time to set it.

To create zones with non-free rate plans, API tokens will require the
billing permissions.

Closes #1101

[simoncpu]

Hi,

If I explicitly set plan = "free" in the cloudflare_zone resource, I still get an error setting plan free for zone error if my API token doesn't have an edit permission for Account/Billing.

ie:

resource "cloudflare_zone" "domain" {
    zone = "example.org"
    plan = "free"
}