[fix][fn] Remove deprecated github.com/golang/protobuf dependency from pulsar-function-go

[mariusgassen]

Motivation

github.com/golang/protobuf is the older, deprecated wrapper around the
canonical google.golang.org/protobuf module. The pulsar-function-go
module retained a direct dependency on it solely for empty.Empty
(github.com/golang/protobuf/ptypes/empty). The canonical replacement
is emptypb.Empty from google.golang.org/protobuf/types/known/emptypb,
which is already a transitive dependency of the module.

Modifications

• pulsar-function-go/pf/instance.go: replace empty.Empty import and
  usage with emptypb.Empty from google.golang.org/protobuf.
• pulsar-function-go/pf/stats_test.go: same replacement in the test
  file's two GetMetrics call sites.
• pulsar-function-go/go.mod: remove the now-unused
  github.com/golang/protobuf v1.5.4 direct dependency.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

The existing TestInstanceControlMetrics test exercises the affected
GetMetrics call sites and continues to pass after the replacement.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment