src: remove unused PROTOCOL_JSON array

Overlooked when moving code around in commit 42da740e ("inspector: split
HTTP/WS server from the inspector".)

PR-URL: https://github.com/nodejs/node/pull/10407


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>