TiDB-operator is unable to delete TiProxy servers

Bug Report

What version of Kubernetes are you using?
Client Version: v1.31.1
Kustomize Version: v5.4.2

What version of TiDB Operator are you using?
v1.6.0

What did you do?
We deployed a cluster with TiProxy and then try to remove the TiProxy from the cluster

How to reproduce

1. Deploy a TiDB cluster with TiProxy enabled, for example:

apiVersion: pingcap.com/v1alpha1
kind: TidbCluster
metadata:
  name: test-cluster
spec:
  configUpdateStrategy: RollingUpdate
  enableDynamicConfiguration: true
  helper:
    image: alpine:3.16.0
  pd:
    baseImage: pingcap/pd
    config: "[dashboard]\n  internal-proxy = true\n"
    maxFailoverCount: 0
    mountClusterClientSecret: true
    replicas: 3
    requests:
      storage: 10Gi
  pvReclaimPolicy: Retain
  ticdc:
    baseImage: pingcap/ticdc
    replicas: 3
  tidb:
    baseImage: pingcap/tidb
    config: "[performance]\n  tcp-keep-alive = true\ngraceful-wait-before-shutdown\
      \ = 30\n"
    maxFailoverCount: 0
    replicas: 3
    service:
      externalTrafficPolicy: Local
      type: NodePort
  tiflash:
    baseImage: pingcap/tiflash
    replicas: 3
    storageClaims:
    - resources:
        requests:
          storage: 10Gi
  tikv:
    baseImage: pingcap/tikv
    config: 'log-level = "info"

      '
    maxFailoverCount: 0
    mountClusterClientSecret: true
    replicas: 3
    requests:
      storage: 100Gi
    scalePolicy:
      scaleOutParallelism: 5
  timezone: UTC
  tiproxy:
    version: main
    replicas: 5
    sslEnableTiDB: true
  version: v8.1.0

2. Remove spec.tiproxy:

apiVersion: pingcap.com/v1alpha1
kind: TidbCluster
metadata:
  name: test-cluster
spec:
  configUpdateStrategy: RollingUpdate
  enableDynamicConfiguration: true
  helper:
    image: alpine:3.16.0
  pd:
    baseImage: pingcap/pd
    config: "[dashboard]\n  internal-proxy = true\n"
    maxFailoverCount: 0
    mountClusterClientSecret: true
    replicas: 3
    requests:
      storage: 10Gi
  pvReclaimPolicy: Retain
  ticdc:
    baseImage: pingcap/ticdc
    replicas: 3
  tidb:
    baseImage: pingcap/tidb
    config: "[performance]\n  tcp-keep-alive = true\ngraceful-wait-before-shutdown\
      \ = 30\n"
    maxFailoverCount: 0
    replicas: 3
    service:
      externalTrafficPolicy: Local
      type: NodePort
  tiflash:
    baseImage: pingcap/tiflash
    replicas: 3
    storageClaims:
    - resources:
        requests:
          storage: 10Gi
  tikv:
    baseImage: pingcap/tikv
    config: 'log-level = "info"

      '
    maxFailoverCount: 0
    mountClusterClientSecret: true
    replicas: 3
    requests:
      storage: 100Gi
    scalePolicy:
      scaleOutParallelism: 5
  timezone: UTC
  version: v8.1.0

What did you expect to see?
TiProxy should be removed from cluster and all TiProxy Pods should be deleted.

What did you see instead?
The TiProxy Pods are undeleted and still in Running state.

Root Cause
In the tiproxyMemberManager's Sync() function, it directly returns if the spec.tiproxy is set to nil. The deletion logic is not implemented.

Start with the tiproxyMemberManager's Sync() function and trace the path taken when spec.tiproxy is nil. Reproduce the issue by removing spec.tiproxy from the provided TidbCluster manifest; done means the TiProxy resource and all TiProxy Pods are deleted rather than remaining Running.