Revert "Avoid data loss with conflicting 'msg' values."

[jsumners]

Reverts #876

This causes issues in dependent applications. See fastify/fastify#2453

To get the same effect, use the log method hook:

'use strict'

const log = require('pino')({
  level: 'debug',
  hooks: {
    logMethod (inputArgs, method) {
      if (inputArgs.length === 2 && inputArgs[0].msg) {
       inputArgs[0].originalMessage = inputArgs[0].msg
      }
      return method.apply(this, inputArgs)
    }
  }
})

log.info('no original message')
log.info({ msg: 'mapped to originalMsg' }, 'a message')

// {"level":30,"time":1596195825857,"pid":48580,"hostname":"mini2018","msg":"no original message"}
// {"level":30,"time":1596195825858,"pid":48580,"hostname":"mini2018","msg":"a message","originalMessage":"mapped to originalMsg","originalMsg":"mapped to originalMsg"}

cc: @markstos

[mcollina]

Could you add that info to the docs as well, maybe in help?

[jsumners]

@mcollina updated.

[mcollina]

lgtm

[github-actions]

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