Add :inets to extra applications and remove :iex

:inets was being used so without the dependency, it was possible to not
have :inets included in OTP releases.

I couldn't find any references to iex nor did I see any reason for it to
be included so I removed that one.

mix.exs

@@ -37,7 +37,7 @@ defmodule NervesHub.MixProject do
         device_api_sni: "device.nerves-hub.org",
         fwup_public_keys: []
       ],
-      extra_applications: [:logger, :iex],
+      extra_applications: [:logger, :inets],
       mod: {NervesHubLinkHTTP.Application, []}
     ]
   end