remove support for `ELASTIC_APM_KUBERNETES_*` envvars

[trentm]

We should consider deprecating and removing the old environment variables in the next major.

from the description of #1041

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Way back when elastic/apm#21 was implemented for the Node.js APM agent, the envvar configuration support accidentally used the ELASTIC_APM_ prefix for the new Kubernetes environment variable support:

KUBERNETES_NODE_NAME | system.kubernetes.node.name
KUBERNETES_POD_NAME | system.kubernetes.pod.name
KUBERNETES_NAMESPACE | system.kubernetes.namespace
KUBERNETES_POD_UID | system.kubernetes.pod.uid

Then in #1041 support was added for the unprefixed envvar names, as originally intended. At the time the prefixed-envvars were kept for backward compat.

[trentm]

Closed by #3536 on the "dev/4.x" branch, which will become the 4.x active branch soonish.