Remove deprecated maintainers field

Due to Hex.pm change, this must be removed to publish hex packages.

````
$ rebar3 hex publish
===> Verifying dependencies...
===> Validator Errors:
     jsn.app.src : deprecated field maintainers found

     Please see https://hex.pm/docs/rebar3_publish for more info.
```

Author: nalundgaard

src/jsn.app.src

@@ -4,7 +4,6 @@
     {applications, [kernel,
                     stdlib
                    ]},
-    {maintainers, ["Nicholas Lundgaard", "Mark Allen"]},
     {licenses, ["Apache 2"]},
     {links, [{"Github", "https://github.com/nalundgaard/jsn"}]}
  ]}.