Remove default value for agent in policy editor

What does this MR do and why?

Describe in detail what your merge request does and why.

Removes the default value (default) for the kubernetes-agent name in the security policy editor to provide more context that the user must submit the name of the agent. The placeholder does a good job!

Some users were confused on what to provide here and had to lookup the policy schema. The word default makes you think of Kubernetes namespaces rather than an agent name.

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