xds: Get rid of xDS v2 dependencies (#10968)

xDS v2 support was dropped about a year ago, but the xds package still
had a few xDS v2 usages. This PR:

- Removes all leftover usages of xDS v2 classes in gprc-xds
- Removes all imported xDS v2 protos and their leaf dependencies:
- Removes xDS v2 generated services
- Makes minor improvements to the xds import script output

### Before
```sh
# Imported 154 protos.
❯ find . -iname "*xds*.jar" -exec du -h {} \; | col -x
  13M ./build/libs/grpc-xds-1.63.0-SNAPSHOT-original.jar
  6.1M ./build/libs/grpc-xds-1.63.0-SNAPSHOT-sources.jar
  388K ./build/libs/grpc-xds-1.63.0-SNAPSHOT-javadoc.jar
  14M ./build/libs/grpc-xds-1.63.0-SNAPSHOT.jar

```

### After
```sh
# Imported 86 protos.
❯ find . -iname "*xds*.jar" -exec du -h {} \; | col -x
  9.1M ./build/libs/grpc-xds-1.63.0-SNAPSHOT-original.jar
  4.1M ./build/libs/grpc-xds-1.63.0-SNAPSHOT-sources.jar
  388K ./build/libs/grpc-xds-1.63.0-SNAPSHOT-javadoc.jar
  9.1M ./build/libs/grpc-xds-1.63.0-SNAPSHOT.jar ```

Reduction:
- Number of protos: 44%
- Jar size: 35%

Author: sergiitk

xds/src/generated/thirdparty/grpc/io/envoyproxy/envoy/api/v2/ClusterDiscoveryServiceGrpc.java

@@ -53,29 +53,19 @@ private static JsonFormat.Printer newPrinter() {
       TypeRegistry.Builder registry =
           TypeRegistry.newBuilder()
               .add(Listener.getDescriptor())
-              .add(io.envoyproxy.envoy.api.v2.Listener.getDescriptor())
               .add(HttpConnectionManager.getDescriptor())
-              .add(io.envoyproxy.envoy.config.filter.network.http_connection_manager.v2
-                  .HttpConnectionManager.getDescriptor())
               .add(HTTPFault.getDescriptor())
-              .add(io.envoyproxy.envoy.config.filter.http.fault.v2.HTTPFault.getDescriptor())
               .add(RBAC.getDescriptor())
               .add(RBACPerRoute.getDescriptor())
               .add(Router.getDescriptor())
-              .add(io.envoyproxy.envoy.config.filter.http.router.v2.Router.getDescriptor())
               // UpstreamTlsContext and DownstreamTlsContext in v3 are not transitively imported
               // by top-level resource types.
               .add(UpstreamTlsContext.getDescriptor())
               .add(DownstreamTlsContext.getDescriptor())
               .add(RouteConfiguration.getDescriptor())
-              .add(io.envoyproxy.envoy.api.v2.RouteConfiguration.getDescriptor())
               .add(Cluster.getDescriptor())
-              .add(io.envoyproxy.envoy.api.v2.Cluster.getDescriptor())
               .add(ClusterConfig.getDescriptor())
-              .add(io.envoyproxy.envoy.config.cluster.aggregate.v2alpha.ClusterConfig
-                  .getDescriptor())
-              .add(ClusterLoadAssignment.getDescriptor())
-              .add(io.envoyproxy.envoy.api.v2.ClusterLoadAssignment.getDescriptor());
+              .add(ClusterLoadAssignment.getDescriptor());
       try {
         @SuppressWarnings("unchecked")
         Class<? extends Message> routeLookupClusterSpecifierClass =