Remove dev environment validation

[Emanuel Medina Gómez]

Hi, an user can have multiple dev environments, and the decision could be in the mix.exs file ,

https://github.com/falood/exsync/blame/ffabe499f85cf19ab13af91937f5f9677f3c62b6/lib/exsync/application.ex#L10

Other option can be

case Mix.env() do
      :pdn ->
         Logger.error("ExSync NOT started. Non`:pdn` environment is supported.")
        {:ok, self()}
      _ ->
        start_supervisor()
    end

[Jason Axelson]

Yeah that makes sense. In 368cbcc I've removed the Mix.env check. In the readme we still recommend only installing exsync for the dev environment so there's still some guardrails against running this in production by accident. Thanks for the suggestion!