provider/google: Remove unsupported region from compute_url_map docs

When trying to run the example code with the most recent terraform
master you get the following errors:

```
2 error(s) occurred:

* google_compute_backend_service.home: "region": [REMOVED] region has been removed as it was never used
* google_compute_backend_service.login: "region": [REMOVED] region has been removed as it was never used
```

Hence remove it from the documentation.

website/source/docs/providers/google/r/compute_url_map.html.markdown

@@ -55,7 +55,6 @@ resource "google_compute_backend_service" "login" {
   port_name   = "http"
   protocol    = "HTTP"
   timeout_sec = 10
-  region      = "us-central1"
 
   health_checks = ["${google_compute_http_health_check.default.self_link}"]
 }
@@ -65,7 +64,6 @@ resource "google_compute_backend_service" "home" {
   port_name   = "http"
   protocol    = "HTTP"
   timeout_sec = 10
-  region      = "us-central1"
 
   health_checks = ["${google_compute_http_health_check.default.self_link}"]
 }