feat: Use open-source Terraform Provider (#403)

This removes our internal Terraform Provider, and opens
it to the world!

cmd/terraform-provider-coder/main.go

@@ -1,13 +0,0 @@
-package main
-
-import (
-	"github.com/hashicorp/terraform-plugin-sdk/v2/plugin"
-
-	"github.com/coder/coder/provisioner/terraform/provider"
-)
-
-func main() {
-	plugin.Serve(&plugin.ServeOpts{
-		ProviderFunc: provider.New,
-	})
-}