Remove default value of label field when agent type is kubernetes

What type of PR is this?

/kind optimization /area devops

What this PR does / why we need it:

As I mentioned in the title, I remove default value of label field when editing Pipeline and select agent type into kubernetes.

See #2986 (closed) for more.

Before	After

Which issue(s) this PR fixes:

Fixes #2986 (closed)

Special notes for reviewers:

Does this PR introduced a user-facing change?

Remove default value of label field when editing Pipeline and select agent type into Kubernetes

Additional documentation, usage docs, etc.:

/cc @kubesphere/sig-devops /cc @kubesphere/sig-console