Remove unnecessary DMAC rewrite from Service CIDR flow

[antoninbas]

Describe the bug
@dumlutimuralp asked a question about the necessity of the DMAC rewrite in the following flow:

cookie=0x1040000000000, table=40, priority=200,ip,nw_dst=10.96.0.0/12 actions=mod_dl_dst:4e:99:08:c1:53:be,load:0x2->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],resubmit(,105)

This action also does not match the ovs-pipeline documentation: https://github.com/vmware-tanzu/antrea/blob/master/docs/ovs-pipeline.md#dnattable-40

It was actually introduced for networkPolicyOnly mode support: #449. However, we no longer support kube-proxy for networkPolicyOnly mode (i.e. AntreaProxy must be enabled), which means that this DMAC rewrite is no longer needed and should be removed:

https://github.com/vmware-tanzu/antrea/blob/fd47097ab7f723650774315092fc6db9b33334d3/pkg/agent/openflow/pipeline.go#L877

Thanks @dumlutimuralp for bringing this to our attention!

Versions:
Antrea v0.9.3 - Antrea TOT

[jianjuns]

This one is missed by PR #1015 when we move noEncap/policyOnly/hybrid to Antrea Proxy. Probably we should compare the flows added by PR #449 and flows removed by PR #1015, to check anything else can be missed.

[jianjuns]

Compared these two PR, seems the DMAT rewrite is the only missed change.
I will push a PR to remove it.