branch-2.7: Remove reference to ProducerSpec from Pulsar Functions GO

[eolivelli]

Motivation

Integration tests on branch-2.7 do not work, because of a bunch of lines of code that were cherry-picked to branch-2.7.

Steps to reproduce

git checkout branch-2.7
mvn clean install -DskipTests
mvn package -Pdocker -f docker/pom.xml
mvn clean install -DintegrationTests -f tests/docker-images/pom.xml

This is the error:

[INFO] [91m# github.com/apache/pulsar/pulsar-function-go/pf
[INFO] pf/instance.go:227:22: undefined: pulsar.DefaultBatchBuilder
[INFO] pf/instance.go:229:45: gi.context.instanceConf.funcDetails.Sink.ProducerSpec undefined (type *api.SinkSpec has no field or method ProducerSpec)
[INFO] pf/instance.go:230:59: gi.context.instanceConf.funcDetails.Sink.ProducerSpec undefined (type *api.SinkSpec has no field or method ProducerSpec)
[INFO] pf/instance.go:233:24: undefined: pulsar.KeyBasedBatchBuilder
[INFO] [0m

Modifications

Remove the lines that do not compile because of missing dependencies (ProducerSpec).

Verifying this change

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

[eolivelli]

@codelipenghui @merlimat PTAL

[sijie]

@wolfstudy Can you review this?