Remove unimplemented endpoints CLI command

[adleong]

The endpoints diagnostic API is currently unimplemented. This causes the linkerd endpoints command to fail with:

$ linkerd endpoints
Error: Endpoints API error: Not implemented
Usage:
  linkerd endpoints [flags]

Aliases:
  endpoints, ep

Examples:
  # get all endpoints
  linkerd endpoints

  # get endpoints in the emojivoto namespace
  linkerd endpoints -n emojivoto

  # get all endpoints in json
  linkerd endpoints -o json

Flags:
  -h, --help               help for endpoints
  -n, --namespace string   Namespace of the specified endpoints (default: all namespaces)
  -o, --output string      Output format; one of: "table" or "json" (default "table")

Global Flags:
      --api-addr string            Override kubeconfig and communicate directly with the control plane at host:port (mostly for testing)
      --context string             Name of the kubeconfig context to use
      --kubeconfig string          Path to the kubeconfig file to use for CLI requests
  -l, --linkerd-namespace string   Namespace in which Linkerd is installed [$LINKERD_NAMESPACE] (default "linkerd")
      --verbose                    Turn on debug logging

We should perhaps remove this command for now.

[olix0r]

This shouldn't be removed; it should be replaced, as described in #2885