Remove incompatible balancing_mode (#1769)

## net-lb-int
* Fix error on apply of example:
```
Error creating RegionBackendService: googleapi: Error 400: Invalid value for field 'resource.backends[0].balancingMode': 'UTILIZATION'. Balancing mode must be CONNECTION for an INTERNAL backend service., invalid
```
* remove unused `balancing_mode` variable, as only one value is possible anyhow

## net-lb-ext
* update in the `backends` description

## net-lb-proxy-int
* update in the `backends` description

## net-lb-app-int
* added validation of `balancing_mode`
* fixed other validations

## net-lb-app-ext
* added validation of `balancing_mode`
* fixed other validations
* removed validation for `locality_lb_policy` as this variable is not used in this module

Closes: #1767

tests/modules/net_lb_int/defaults.tfvars

@@ -6,7 +6,6 @@ vpc_config = {
   subnetwork = "default"
 }
 backends = [{
-  balancing_mode = "CONNECTION"
-  group          = "foo"
-  failover       = false
+  group    = "foo"
+  failover = false
 }]

tests/modules/net_lb_int/forwarding-rule.tfvars

@@ -6,9 +6,8 @@ vpc_config = {
   subnetwork = "default"
 }
 backends = [{
-  balancing_mode = "CONNECTION"
-  group          = "foo"
-  failover       = false
+  group    = "foo"
+  failover = false
 }]
 global_access = true