Timestamp should not be included as field in JSON parser

[danielnelson]

Relevant telegraf.conf:

[[inputs.file]]
  files = ["test"]
  json_time_key = "timestamp"
  json_time_format = "unix"
  data_format = "json"

System info:

Telegraf 1.9 master

Steps to reproduce:

Input data:

{
    "timestamp": 1541183052,
    "value": 42
}

1. telegraf --test

Expected behavior:

file value=42 1541183052000000000

Actual behavior:

file timestamp=1541183052,value=42 1541183052000000000