revert: "fix: reject requests targeting a cachew playpen when not run…

[alecthomas]

…ning as one (#204)"

This reverts commit 2f3c5c3.

@jrobotham-square I'd prefer not to have anything bespoke in Cachew. We have OPA enforcement now, we can add an OPA policy to do the same thing:

opa {
  policy = <<EOF
    package cachew.authz
    
    default allow := false
    
    allow if {
        permitted
        not deny
    }
    
    permitted if input.method == "GET"
    permitted if input.method == "HEAD"
    permitted if startswith(input.remote_addr, "127.0.0.1:")
    
    deny if {
        data.IS_PLAYPEN != "true"
        contains(input.headers.baggage, "cachew-playpen=")
    }
  EOF
  data = <<EOF
    {"IS_PLAYPEN": "${IS_PLAYPEN}"}
  EOF
}

We can extend the OPA integration to support returning a message or a different HTTP status if required.