Drop logic for `peer_cidr_blocks` autocalculation

[dkropachev]

Currently we construct Meta.GCPBlocks from blocks.txt that we took from https://cloud.google.com/vpc/docs/subnets#ip-ranges And use this region/cidr mapping for calculating default CIDR for GCP VPC Peering.

Idea is to drop this logic entirely and force client provide CIDR.

There is one case that is going to be broken with this change, is when GCP VPC is created with auto_create_subnetworks:

resource "google_compute_network" "app" {
    name = "app"
    auto_create_subnetworks = true
}

It is broken due to absence of google_compute_subnetwork in the state and inability to import these resources automatically, which should be handled by GCP terraform provider.