[XdsInteropClient] Remove incorrect behavior around keep-open (#38613) #38632
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Backport #38613 to v1.66.x
Based on the spec and the reference Java implementation, it's the server that needs to honor the "keep-open" behavior and not the client.
(I'm not changing the server's behavior in this PR, since we are not using C++ servers and instead using Java servers for the concerned tests.)