tidb-operator fails to delete discovery annotations and labels according to updated CR

Bug Report

What version of Kubernetes are you using?

Client Version: version.Info{Major:"1", Minor:"23", GitVersion:"v1.23.1", GitCommit:"86ec240af8cbd1b60bcc4c03c20da9b98005b92e", GitTreeState:"clean", BuildDate:"2021-12-16T11:41:01Z", GoVersion:"go1.17.5", Compiler:"gc", Platform:"linux/amd64"}
Server Version: version.Info{Major:"1", Minor:"22", GitVersion:"v1.22.9", GitCommit:"6df4433e288edc9c40c2e344eb336f63fad45cd2", GitTreeState:"clean", BuildDate:"2022-05-19T19:53:08Z", GoVersion:"go1.16.15", Compiler:"gc", Platform:"linux/amd64"}

What version of TiDB Operator are you using?

pingcap/tidb-operator:v1.3.2

What storage classes exist in the Kubernetes cluster and what are used for PD/TiKV pods?

default

What's the status of the TiDB cluster pods?

What did you do?

We first added a label key: value to the discovery service, the label is successfully added to the discovery service. Then later we wanted to remove that label from the discovery service by changing the CR, we found that tidb-operator cannot remove the labels.

What did you expect to see?
labels and annotations in spec.discovery can be added and removed.

What did you see instead?
The tidb-operator fails to remove labels and annotations from discovery service when users specifies the removal in the CR.

Start by reproducing the issue with a discovery service label or annotation added through the CR and then removed from it. Trace the reconciliation entry point that updates the discovery service; done means labels and annotations absent from spec.discovery are also removed from the service.