[bazel] `proto_root` was appended to `dir_out` earlier, so don't duplicate it here. (#37990)

This will resolve a bunch of warnings we're seeing in Envoy, of the form:

```
INFO: From Action external/com_github_grpc_grpc/src/proto/grpc/reflection/v1alpha/reflection.grpc.pb.h:
bazel-out/k8-opt/bin/external/com_github_grpc_grpc/external/com_github_grpc_grpc: warning: directory does not exist.
INFO: From Action external/envoy_api/envoy/service/ext_proc/v3/external_processor.grpc.pb.h:
bazel-out/k8-opt/bin/external/envoy_api/external/envoy_api: warning: directory does not exist.
INFO: From Action external/opencensus_proto/opencensus/proto/agent/trace/v1/trace_service.grpc.pb.h:
bazel-out/k8-opt/bin/external/opencensus_proto/external/opencensus_proto: warning: directory does not exist.
INFO: From Action external/com_google_googleapis/google/devtools/cloudtrace/v2/trace.grpc.pb.h:
bazel-out/k8-opt/bin/external/com_google_googleapis/external/com_google_googleapis: warning: directory does not exist.
INFO: From Action external/com_github_grpc_grpc/src/proto/grpc/reflection/v1/reflection.grpc.pb.h:
bazel-out/k8-opt/bin/external/com_github_grpc_grpc/external/com_github_grpc_grpc: warning: directory does not exist.
```

```
bazel-out/k8-opt/bin/external/com_github_grpc_grpc/external/com_github_grpc_grpc: warning: directory does not exist.
                    |----copy-1-from-dir_out-----||---copy-2-from-proto_root---|
```

Closes #37990

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37990 from asedeno:warnings 801da4c5cd
PiperOrigin-RevId: 689517449

bazel/generate_cc.bzl

@@ -134,7 +134,7 @@ def generate_cc_impl(ctx):
 
     # Include the output directory so that protoc puts the generated code in the
     # right directory.
-    arguments.append("--proto_path={0}{1}".format(dir_out, proto_root))
+    arguments.append("--proto_path={0}".format(dir_out))
     arguments += [_get_srcs_file_path(proto) for proto in protos]
 
     # create a list of well known proto files if the argument is non-None