Replace the dead env var replace action with delete

[joetannenbaum]

environment:variables --action=replace called PUT /environments/{env}/variables, which the API removed. It was swapped for an atomic POST /variables/delete, and nothing in the CLI used it.

--action=delete now takes its place. --key accepts a comma-separated list, and interactively you pick from the environment's existing keys. Deleting goes through confirmDestructive(), so it needs --force when non-interactive.

Kept this on the existing --action option rather than adding a command, since a delete verb would have meant the first three-part command name in the CLI.